is it safe to register a listenner twice ? EG Bangle.on('GPS', processFix)

@HughB I guess this is mainly for the 'Kitchen' app? Just a thought but your original motivation was to avoid turning GPS off/resetting compass? Since 2v10 or so that hasn't been an issue (GPS is kept on for ~1 sec even when new apps are loaded, in case they ask for it again) so if you're finding you are fighting the system might it be worth looking at revisiting using separate apps?

What happens to event listeners and set intervals on Bangle.js, when the app is changed to another one (and therefore quit)? I suppose all listeners and intervals are cleaned up. Is this right?

Yes - Bangle.js actually cleans everything up - which is a cause of frustration to some users, but it does make things a lot easier and more reliable. You know when you start a new app it's always going to start in the same state, and not run out of memory because of any app you'd run previously :)