We have no idea and there’s no real way even in principle for us to know.

They can still pull off an MITM attack by sending the MITM’s keys (pretending to be the other user’s keys) because they control they key exchange.

It’s an active attack and can’t apply retroactively but within these constraints they can still do it.

Yes, if they add a wiretapping key to one or both of your key lists, which is silent/invisible to the sender.