A bank transaction never settles. Though unlikely, it is possible that quantum events may sporadically reverse the transaction.

Both are probabilistic and highly unlikely.

>Both are probabilistic and highly unlikely.

A simple case of this happening with bitcoin is if the network fragmented. For example if a country had a firewall which temporarily blocked bitcoin. The country would continue slowly adding blocks which would likely revert when they reconnected back with the rest of the network.

People would only mine on the shorter blockchain if they think it's valid and good luck adding a country firewall in an undetected fashion. It will be directly visible in one of the two forked blockchain that a lot of the hashpower has vanished.

If a country is behind a firewall, most likely, almost no new blocks will be mined because the hashrate difficulty will stay constant while the computational power behind the firewall will become too low. Blocks will be mined much more slowly for a period of time inversely proportional to the hashpower behind the firewall. Most likely, that chain will enter into a "mining death spiral".