Then everyone plays "good" wrong in CF. No one is fully selfless. Also just because he attacked a celestial creature (which I'll get to later) doesn't mean a shift in alignment. You could just as easily argue that the conjurer looted the body of one of the light, a pledge no less. Would you argue that every good tribunal who strikes at a maran striking in town should be align-shifted? Certainly not. Good shouldnt be so black and white in my view, just as it is in real life. Emotions, frustration, anger, they all play a role in a person's actions. And a person can act irrationally and lash out in anger, but if they realize their mistakes and truly regret their actions, they can still be a "good" person.
I hate to use religion as an example, but someone like King Solomon from the Old Testament made mistakes where God told him he fucked up good. Yet, God still considered King Solomon to be a good person. And as a preemptive strike to anyone, don't fight the hypo by arguing that christianity is bad, god doesn't exist, etc. You're just attacking part of the hypo that doesn't actually matter to the core argument. Attack the argument itself.
As for the cherub strike, I think it's tough because you're in a position where the conjurer controls the cherub. The cherub is not a divine being of its own accord, even if someone tries to RP it that way. Why? Because then you're giving free reign for the cherub to do anything they want, like trying to kill elves, attack the watcher, etc. without anyone retribution. You could just argue, "oh, but it's a celestrial creature, he can do no wrong." Homun asked the cherub to leave his guild. The cherub should have left his guild. It's sort of (ok, not really, but sort of) like an elf tribunal telling a criminal maran to not enter Galadon. Maran enters galadon. Should the tribunal just say oh, well, can't do anything about that? No. Does that make him evil/neutral if he tries to run off the maran? No. That's not to say i would have played a goodie like Homun did and strike the cherub, but i don't think it necessitates an alignment change either.