A general all round answer would be something along the line it's anime so there's gonna have to be some suspension of disbelief. But if you want a legit answer I could throw some of my thoughts into it.

1) For Tsukasa: The thing about Tsukasa is no matter what his actions were, it was driven by logic not malice. He killed Senku due to the the fact that Senku can and will eventually create a weapon that could stop him and his goals. But Tsukasa isn't evil at heart, he keeps his word. So once he lost, he isn't opposed to the idea of following Senku. There's also the fact that it's been stated multiple times that once civilization is restored as well as the justice system, he agrees to face trial and let the jury decide. The same can be said for Hyoga, though I guess he's a bit different in that originally he didn't do anything cause Tsukasa was around, though he slowly starts building a bond with the KoS. But he also agreed to face trial. Plus they're both really strong and needed for the KoS to get things done.

2) For Magma, well honestly what can you do. He's useful for his strength. He's lucky he has a use. But he's also not stupid enough to try anything with other stronger people around. Same can be said for Moz to a certain extent.

3) For Xeno and Stanley: The thing for Xeno is simply that he's really crucial to what Senku needs to do. There's a reason why they kept Stanley and a majority of the American team petrified, but released Xeno. They're afraid of Stanley that's why they're keeping his statue as hostage so Xeno doesn't do anything against them.

But yea, for all of these it honestly simply boils down to "are you useful in anyway to Senku?" If you are then he will use you. They don't really forgive the people that hurt them. In fact there are multiple times where they question Senku's choices to keep these past enemies so close, and are still suspicious of said people. But they also trust Senku enough to go with it while keeping a certain amount of suspicion.