I've been looking into this for a while now but there seems to be old solutions that focus on mainly calling frege from Java. Is there a way to write an complete app/Activity purely in Frege? If not, why?
You might look here, it's most recent. http://mchav.github.io/frege-on-android/
Don't be shy to contact the author directly.
The short answer to your question: It is possible to write a complete App in Frege, except for a bit of glue code that implements the interface android.app.Activity. The reason being that we can't implement interfaces in Frege.