This is a very interesting hypothesis. My only issue with it would be the motivation to locate or find April in Arcadia. My reason for saying that is becuase the people that knew of April's ability to shift or the existence of Arcadia were the two Kin, which disappear/go into hiding/die (the latter being an assumption/insinuation); Emma and Charlie (Emma originally didn't believe her story and Charlie did but they're her friends and wouldn't have gone off telling others of April's "discoveries"); Gordon and Adrian (both of whom we last see in the Guardian's realm)...assuming Adrian (the guardian who stepped down after 200 or 300 years overdue - went back to Stark PTSD unlikely candidate to divulge the score) and Gordon is the 13th guardian still in place -unable to divulge. There is that Pastor or Priest at Hope Street but again, the motivation for him to spill anything about Arcadia, the Vanguard or the Sentinel to random people or uninvolved parties is unlikely. Also April no longer posed a threat to any Stark borne initiative. Besides the April thing, I find this hypothesis brilliant.
This'll be a long one, sorry about that. Lots of information and still not all of it!
I'll start off with addressing what you said about the current guardian, Gordon Halloway. A guardian of the Balance wouldn't be able to divulge anything, even if they wanted to because their hope and dreams is locked inside the disk, inaccessible during the 1000 year period they serve as guardian. This will allow the person to focus entirely on the balance and redistribute chaos and logic as is necessary. This has obviously not happened to Gordon since he can step down and speak to April.
Remember how there are three trials that every potential guardian must pass in order qualify and gain access to the tower? Gordon were never subject to these tests because he snuck in after April had passed them, so what if he wasn't qualified even after April made him whole again?
After Gordon was installed as the new guardian, the Collapse happened over in Stark and suddenly there is science and technology popping up in Arcadia.
Remember how April was the fulfillment of a handful of savior prophecies in TLJ? Have a look at the Venar one:
"One who will bring order to Chaos, only to release Chaos on the innocent. One who will restore the Balance, only to finally break it."
I think it speaks for itself. As for the 'usual suspects' that may have an issue with April involving herself in their business, I have the following:
He knows that April is a shifter, the existence of Stark, Arcadia and Storytime. As for motivation, he is tainted by the Undreaming and he is human.
Crow told Zoe many things about April abandoning him 8 years prior, including this:
Crow: 'On top of that, the Azadi were after everyone from Stark, everyone with special powers, everyone who was allied with 'the magicals'.'
April may not be a threat over in Stark, but she is in Arcadia. She has joined up with the rebellion, who are apparently quite skilled at sabotaging the Azadi projects. The Azadi obviously doesn't know that she is 'The Scorpion' but what Crow told April they know a lot more than what we've been made to believe.
In the first April-sequence in 'Dreamfall' April is on her way to Marcuria with Chawan, Na'ane and Brynn, let's have a look at some of the dialogue:
Chawan: Our sources in the city may have information.
April: I hope so. But we need to focus on getting the supplies and bringing them back to the swamplands. Everything else is secondary.
Na'ane: They are all depending on us. If another moon passes, the children will be at risk. If they should fall ill...
Chawan: Na'ane is right. We go back as soon as we have what we came for. And we do not engage the enemy within scouting distance of the city.
April: We'll be cautious, Chawan. Now we continue. Another few days due south, and we'll see her. Marcuria.
Brynn: I can't wait to see the city! If it's anything like you've described, April, it must be amazing.
April: It's been a long time. Things change. Come on. Let's move out.
April hasn't been in Marcuria for a long time and doesn't plan on sticking around long either, but in a few days she'll be in Marcuria for a little while, at least. Since nobody appears to have seen her for 10 years (8 years for Crow), This seems to be first time she returns to Marcuria after TLJ.
Since it appears as if she is second-in-command, I suspect she has been with the rebellion for more than just a couple of days, she may even be 'co-founder'. Brynn mentions that they have informants in Marcuria, so they DO have people who knows things. Benrime Salmin, for example; She is a Rebel sympathizer who provides room and board for rebels visiting Marcuria.
When Zoe has her first shift to Arcadia, April has already arrived. We know this because Brynn is there eating breakfast and he knows something is up because the Journeyman Inn i closed for business and Zoe's sad habit of name-dropping without discreetion:
Brynn: Benrime? Visitor.
Benrime: What are you on about, Brynn?
Brynn: She's standing right next to you.
Zoë: Um, hello?
Benrime: Oh! Blessed be the Six, child, you startled me!
Benrime: How did you get in? We don't open until midday.
Zoë: Open what?
Benrime: The inn, child? What do you think? Are you soft-witted or...
By the Balance, you are from...from the other side, aren't you?
Zoë: You don't understand! I have absolutely no idea what's going on! I'm almost starting to think that those stories Charlie told me about April Ryan are true.
Let's stop for one second and talk chorology and timeline. Not all parts are in chonological order cause if they are, then we're dealing with a huge continuity issue that is a game-breaker:
After the April introduction sequence we're back to Zoe who arrives in Newport, Venice. As you may recall later that same day she is hooked up to the Dreamachine prototype and ends up in Marcuria after Faith has booted her out of Winter. Since Brynn is already in Marcuria when she arrives, April's intro sequence doesn't fit in because of what she told Brynn: "Another few days due south, and we'll see her. Marcuria." So since the game started two days earlier(11:30 on January 21st 2219), the April sequence occurred several days before the game even started. This opens up the possibility that more sequences may be in the wrong order, and/or give the impression that simultanous events occur at different times. With that in mind, Kian Alvane time!
In the first Kian sequence, he is instructed to travel to Marcuria to locate and assassinate 'The Scorpion' posthaste, I'll edit the dialogue a bit to make it tidier since the Six have the annoying habit of splitting everything into bits so everyone gets to say something:
The Six: We have just had word from our armies in the East, and it worries us deeply. Our dearest Apostle, we have a very important task for you.
Kian: Yes, Mistress.
The Six: Apostle, you know how some Eastlanders still resist our noble efforts at uniting their fractured nations and bringing the Word to their Goddesless lives. The Mission continues unabated, of course. Nothing shall stand between the Glory of the Goddess and those lost ones who have strayed from her light.
But still there are a few who have chosen to oppose us. Those misguided pagans have become a dangerous obstacle in the Path. Apostle Kian, we need you to travel East to those heathen lands to bring the Word to the heretics. There is a man who leads these rebellious few, these insurgents. A man whose hands are soiled with the blood of many Trueborn. A Daemon.
You must travel tonight to the havens, Apostle, and board a cloudship bound for the city of Marcuria. When you reach your destination, you will seek out this man, this...this butcher. They call him 'the Scorpion'. You must seek him out, and you must convert him.
They obviously don't know exactly who they're looking for, but they appear to know something since they're sending him to Marcuria after they heard from their armies in the east. Since April had already arrived in Marcuria at this point someone must've leaked information to the Six that members of the rebelion had arrived in Marcuria, you don't send your most devout and skilled assassin unless you've got something solid to go on.
When he leaves, the Six has this to say:
Yona: Can we trust him?
Maha: If not him, then who? He is our most faithful Apostle.
Abya: He knows not to question our word. He will complete his mission.
Maha: The Prophet was adamant. There is much resting on this task. It is the last obstacle, the Prophet said. When it is done--
Abya: The light of the Goddess will wash across the world like a tidal wave, drowning all those who lack faith.
Yona: May that day come swiftly. And may the Goddess aid the Apostle in his mission. The future of our Empire rests on his shoulders now...
Remember the part when April climbs the scaffolding and listens in on the conversation between the Prophet and Sister Sahya? Since Sister Sahya can clearly see his face since she is standing right in front of him, why the disguise and why whisper? He either knows someone is watching, or is taking precautions in case someone is. Why would he go through the trouble of doing that if nobody knows who he is? I think this is someone is a well-known face to the Marcurians, and likely the rebels too. Since he was the one that requested for Kian to be sent, without consulting Sister Sayha even, he must know about members of the rebelion having arrived in Marcuria.
Then there's the connection between Brian Westhouse and the Azadi:
Brian tells both Benrime and Zoe that he visited the Azadi Empire after TLJ ended:
He told Zoe: "I passed as a traveler in their lands. It's not closed to outsiders, and as long as you respect the Azadi and their customs, they give you passage. It's such a different culture from what we're used to, but they are an interesting people, and I found my visit to be highly...educational. What they're doing in Marcuria, though... It's certainly controversial. On one hand they brought freedom and peace. On the other hand... They've rounded up the magicals, stuck them in a ghetto and God knows what they intend to do with them next."
He told Benrime: "I wouldn't know what the Azadi are like here in Marcuria, but in Sadir they were reserved yet friendly. And as a man, I had less freedom than a woman. Still, I encountered no animosity. People took me in, fed me, told me about their history and religion. I learned a lot. Whatever the Azadi have done here, they are not cruel or evil. They do put a lot of faith in their Goddess and their Empresses, yes. But it's heartfelt."
Azadi sympathizer. Maybe that's how he was able to offer Zoe and Crow passage to The Dark People's city on a cloudship owned by the Azadi? When April asked him about the whereabouts of the White Dragon Reborn, he had this to say:
Brian Westhouse: "I may know where to find her. I met a fellow traveler - a Dolmari from the Rose Court, actually - on a ship from Ge'en to Irhad, not long ago. He told me he'd been to the Dark People's library. Not many people are allowed there, and it was a great privilege. While he was there, however, he said that he was introduced to a pale, white haired woman who introduced herself as the White of the Draic Kin. Now, I don't know if she's still there, or how you'll get to the Dark People's City, but it should at least get you on the right path."
Apparently he doesn't bother to offer April a ride on the Azadi-owned cloudship. Lucky for her that there's a Shadowship docked in the Marcurian harbor at that same exact moment, and let's not forget the reason why the White Dragon Reborn went there to begin with:
White Dragon Reborn: "I'm in exile. Someone hunts the Kin. That's why I'm here, and why I'm wearing this skin. It's tight and uncomfortable, but it conceals me, and it makes some humans look at me with admiration."
I have a hard time believing his Dolmari traveller story, if the White Dragon Reborn feels the need to both hide out in a high-security place and disguise herself as a human, would she really introduce herself to random people?
Then there's this unforgettable moment:
White of the Kin: You sound so familiar.
Brian Westhouse: You've heard my name before?
White of the Kin: Your presence, your being, sounds familiar. And also...also disconcerting. I feel a prickling on my skin. What do you call it? Goose bumps?
I've got a lot more to say about Brian Westhouse, especially when it comes to how technology appeared in Arcadia when it shouldn't be there at all. But I've gone on long enough so I'll save it for another post, assuming you're interested