There just seems to be a few things stacked against OP. I'd just divorce him and move on with my life.
If the husband did do something shady, then this is such a terrible look for USCIS for missing it at I130 level, Consulate, ROC and then N400; that is a lot of hurdle to cross with a fake divorce decree. A regular citizen found what a multibilllion dollars state department couldn't find 4 different times? Hmm hmm.
Your 7th point is very key. If he ever petitions, then the scrutiny starts all over again, perhaps he will get caught this time, or not. Who even knows anymore.
Also, point 4, I wouldn't trust many of the lawyers back there, what if they told you what you wanted to hear for some $$?
Anyways, if he indeed perpetrated this evil, I hope he gets caught. He has to. He must.