I'm actually not a poly - I've been reading this subreddit for about an hour - and first off, I have to say that everyone in this poly community is of extremely high intelligence, resilience, and you all sound like amazing people. I think it may be true what someone posted in another thread - being poly is like being the expert level at relationships. It seems to me that involves operating at a high IQ and high EQ, so my hat is tipped to you guys.
I hope it's not totally inappropriate if I respond to this one, so forgive me in advance if I've overstepped the line. However it seems to primarily involve a fundamental relationship issue. The OP is struggling because their partner is struggling with something OP can't help with. This crops up now and again in life, and I think that in every case it's frustrating but you just have to be there for that person while they figure it out. It will be a tense time but your boyfriend won't forget why he adores you in the meantime. You're still the same person.
He needs to think for himself, and I hope he doesn't choose the path that so many married couples do of staying together for the sake of the marriage. It's impossible for some to conceive of this institution breaking down, and they're even willing to forgo what they want most to save it. Unfortunately there's a lot of bad marriages out there ("we're staying together for the kids" is my favourite - wish my parents hadn't) because people hide their inner desires and wishes and just march along grimly. Both individuals are unhappy and inflicting it on those around them.
I hope your boyfriend is smarter than this. If he's poly then he's poly and hopefully he'll realize that. Divorce is very tough but the marriage as it stands consists of two people who want very incompatible things. "Irreconcilable differences" couldn't define it better.
Good luck. As to your level of involvement, I'd just sit at the level you feel comfortable at. Supportive of him, but do not allow yourself to feel taken advantage of by either J or B.