The main point I wanted to make is that if you only assume the best but don’t actually believe it, you might think their reaction to your (pretended) niceness “confirms“ your assumption, where what it actually confirms is your belief and not their original intent.

Yes, knowing anything for certain is difficult so it's worth always harbouring a little doubt about both positive and negative conclusions.

Dzerzhinsky liked to say "Trust but verify", and I think that sums it up - if you look at the world entirely from his point of view, head of the Soviet secret police, then you will suspect everyone and if you act on that you'll end up hurting lots of innocents in amongst the people who would really do you harm. If you're in a situation where this is a good survival strategy it seems to me that one should try to leave rather than play the game.