To summarize what we know:
1. April shifted to the Guardian's realm with the help of the kin.
2. April can't shift anywhere on her own
3. Dreams flow to Arcadia
4. April doesn't believe in herself
5. The white kin seems to suggest April should be able to shift on her own, but can't for unknown reason. This is to say April CAN shift with help
This brings up a question, would she be able to shift to Stark with the help of the kin? What exactly did the kin do to help her shift?
April's ability to shift to the Guardian while it shouldn't be possible is another issue that is not directly connected to her ability to shift in general.
Ok, new theory:
The flow of dreams is like a current in a stream. You can't cross the stream with strong current without help, but with help you can do it.
So the flow of dreams is not blocking her ability to shift, just making it a lot harder than it is.