Thanks! To answer your questions:

Currently the app uses its own address book. You're right that it might be useful to integrate Bitmessage addresses with the standard contacts list - I'll look into it. Thanks for the suggestion.

I haven't tried using Bitseal as a background service with an API yet, but I think it should certainly be possible. One good point about Bitseal's code is that there's a fairly clean separation between the UI and the background processes, so it ought to be easy to either adapt Bitseal itself or just pull out the background code and make it into a standalone app.

The only major change I made to PyBitmessage was a new set of API commands for the functions required by the Android app. Other than that there were some very small tweaks, of which only one was relevant to normal PyBitmessage. I've submitted that as a pull request: https://github.com/Bitmessage/PyBitmessage/pull/709