Could a mobile phone or powerbank connected to each other and stored inside a backpack overheat or catch fire?

It is possible but Not guaranteed to happen to You.

Still, I would avoid packing a powerbank in any place where it couldn't easily ventilate,

and I would Never connect one to anything else while it was in a confined space.

Pack them safely at the top of the contents of the bag and so that the connecting cable can't come out and and it should be OK. I charge one in a car glovebox like that and it's fine. Best not left unattended though as any battery device can get too hot when charging.

Yes. Those are actually the most insidious kinds of fires. They usually smoulder first, and then start to burn, and unlike fires caused by ignition sources such as gasoline, are both harder to detect at first, but are harder to put out, kind of like pancreatic cancer (few symptoms at first, but almost always fatal vs testicular cancer (fairly obvious at an early stage, and fairly easy to cure). In any case, if you do this, take it out to air for a few minutes every hour.