No one could have predicted that the Mind Stone would do that to the Ultron program, and when his guilt over this showed up in CW it, like his ego, grew out of control to support the Accords.
While the program becoming genocidal was unpredictable, Bruce and, Tony knew, the others already did worry that it could go wrong and be misused. After it does go wrong he insists on making something bigger and more powerful without objections. If his guilt over it came later in CW that seems like at best deflection (claiming the whole group was reckless and indifferent to bystanders) and at worst that he really didn't learned anything from it (he justified creating Ultron on that in the best-case scenario there would be more security and lesser- and worst-case scenarios and abuses shouldn't be considered, likewise again he was trying to marginalize worst-case possibilities (although I guess he thought governmental authority would happen anyway if it was not accepted but it's not clear why accepting it would make it better)). And it's not clear by the end he thinks or we should think signing the Accords was a mistake, the guilt excessive or misapplied.
The others said nothing because they all felt some degree of collective guilt too.
I don't see why they should feel particularly guilty when, from what we were shown, the charge was destruction in general but the destruction had been and, if they hadn't been there, would be caused by the villains as much or more than it was (or there would be domination). The one time they weren't just defending from already and otherwise existing threats, where they did cause the problem, only Tony and Bruce caused it so it makes sense that the public and authorities (and film) should have been more focused on that and who was responsible.
He worked with Ross because Ross WAS the Secretary of State, and Bruce wasn't around.
I don't think that says much for his loyalty or ethics/judgment of character, if he'll support him despite how bad he was to Bruce (villainous and even reckless, although in CW he seemed a bit less unreasonable though still ruthless) he'll follow pretty much any authority.
Ant-Man joined up because either way he was in trouble because the Accords meant people would be coming for him no matter what.
It didn't feel that way although it wasn't totally clear what the Accords meant; it seemed to apply just to the Avengers, local superheroes (of which there didn't seem to be many) would remain illegal but not more illegal than before and there was no indication there would be "drafting" (on contrary people would be able to leave & retire if they disagreed strongly enough). It seems like he was willing to join a fight/mission without knowing much about it, back Captain America against Iron Man because he liked CA a lot and IM less or not at all and didn't mind breaking the law and becoming a fugitive, saw it as old so not a big deal.
If the Accords did/would apply to all local heroes Spider-Man is a hypocrite and even dense for wanting to have it and enforce it against others (and generally thinking that Tony Stark does and the UN controlling heroes will look out for the little guy) but not sign it or be bound by it himself.