Home button stoppend working after opening screen

No they dont need a new motherboard they just need to reprogramme the phone for another button pairing them up. Yes only apple can pair a new button but no it doesn't involve a new motherboard.

Other than that yes heat or damage to the flex is the cause also it might not be the button the button might have survived there could also be a possibility u damaged the flex on the screen instead and a replacement screen would fix that, so it's worth trying a new screen anyway.

Also if it's not too bad there are other people that may be able to look at the button for u and hopefully fix it.

Up until now I have tied reseating both the homebutton connector and the screen connectors, but without result. Also, both cables don’t seem to be nicked or damaged in any way, so if possible I really think the heat has damaged the homebutton. Lastly, I will try a new screen when I’ve received it.

How hot was the heat being outputted to the phone? Is this a hair dryer or a heatgun? If the home button area hasn't been touched I say the home button is fine because heat wasn't being directly blasted onto the ribbon cable of the home button itself.

Almost always it's just the screen ribbon cables going bad, a replacement screen will fix this problem if it's the case.

Had it happen once on an original screen after opening the phone, bad luck it seems.

I have received the new screen assembly, and the home button works agains! So apparently something in the old screen assembly connecting the homebutton broke due to the heat.

Now one more question: when I closed up the phone I noticed a screw sticking to the earpiece. It is a very weird looking screw and I have never seen it before. Where could this come from?

If it's not a part of the front camera assembly, I'd check the camera bracket and surrounding brackets. If it looks like a screw you've never seen before, could it possibly be a standoff screw?