I spoke to two lawyers who said I don't have a case, thanks to my ex-FIL's many years of enabling, hiding, and machinations. I have no proof of anything untoward, and our courts would grant me no more limits than what I currently have, a limited visitation schedule with supervised visitations. They were both disapproving of the bedroom situation, but advised to let this play out. Short of plunking down $10k I don't have on "discovery" with unknown outcomes, my best bet is to keep DD8 in counseling, maintain a working relationship with my in-laws, and sit on my hands until he gets out of rehab and relapses again, which he statistically will. So, that's what I'm doing for now.
[Meanwhile, his parents have decided - again - that I am a terrible, no good person and have cut off communication with me for the crime of... encouraging him to get into treatment per their request.]
I'm taking all of your input into consideration and will likely be posting more here as I'm trying not to get sucked back into this addiction vortex.