Nothing stopping Iran from joining the ICC. Except that the investigations would go both ways.

Palestine is a member and we all saw what happened there. The US has personally threatened the judges.

I mean, the ICC did issue a warrant for individuals on both sides of that conflict and seems willing to prosecute. The Palestinian national died in the conflict, so obviously could not be prosecuted. The Israeli nationals are regrettably refusing to surrender themselves, which would be an issue no matter which body attempted to prosecute (unless it was a domestic Israeli court). That is hardly a situation unique to this conflict - lots of people with warrants attempt to evade capture.

The US behaviour is despicable, but ultimately it hasn't really changed anything.

Yes, though one without consequences. Until the next guy comes along and actually enforced it.

Of course it is. Is a rapist innocent if he gets away?