notifications (default and fullscreen)

There will only ever be one notify file on the device. But there would be a unique info file on the bangle

Yes, that's the problem

Would it not be possible to have one notify module and have it configurable fullscreen / pop up style notifications ?

Yes, and it may have been easier. However the idea was to allow other developers to make different styles of notifications that could be installed, so you had a choice - it's just that nobody has actually done that yet.