Adding "back" button to Weather app (matching Agenda, Health, etc)?

That's odd. On Bangle.js 2 this works great with button and touch as you say:

Bangle.setUI({mode:"custom",back:function() {
      Bangle.showClock();
    }});

Maybe we could add similar code to the Bangle.js 1 setUI
But I suppose that would lead to the same problem of it not working in older firmware...
On the other hand: it doesn't work now either

Yes, I think that's the best bet. We need to fix that or setUI({back}) on Bangle.js 1 ends up being a bit useless. Plenty of stuff uses setUI({back}) even now so it's something that should probably be tweaked.

Not sure what you think, but since long-pressing BTN3 goes back, it might be the best bet to use that instead of BTN2? Not sure...