Divorced Parents. Help with Mother's Husband (invite or not).

Sorry if this is a bit long, but there is a lot to unpack to give you guys an idea of the serious pickle I'm in (along with FW, but it's not as much her issue, and she's firm in standing by whatever decision I end up going with) . Brief rundown on the background of everything. My parents split about 9 years ago, right when I was wrapping up college. From what I gather my dad screwed quite a bit of stuff up, and became emotionally attached to his best friends widow, so my mom couldn't take any more and left him. She met a guy, they started seeing each other and my dad started trying to "get her back" or at least have a period of time to work on things, in which her BF told my dad in so many words to go pound sand. At one point there was an altercation where my dad was pissed off trying to confront them (possible threats) and the BF had called the cops, in which my dad was precautionarily arrested. Needless to say there is bad blood there. Fast forward, my mom decided to marry this guy despite none of my siblings or even her friends really liking him. He's not the worst guy in the world, but does seem a bit manipulative and rubs you a bit the wrong way, it's hard to put a finger on. So anyways, my dad can absolutely not stand this guy, like can not be in the same room, and there have been essentially 0 interactions between my dad and moms husband all these years except 1. My mom made sure "everyone" was invited to my brothers wedding 2 years ago, and it did not go well. My mom and her husband were there and my dad as well (he has not had any sort of significant other all these years). Whether he was or not my dad felt like my moms husband was taunting him, he got super pissed off couldn't control himself and chose to see himself out not long after the reception started before there was an altercation. So now 2 years later lucky me gets to deal with the same crap for my wedding, and we are needing to get our invites out ASAP. Side note, I have an excellent relationship with both my mom and my dad, but obviously completely separate and totally different relationships as they do not talk. As for the invites my dad has made it pretty clear that he will either not go (along with my aunt and uncle who are very much on his side) if my moms husband is coming or if he does force himself to go in that case it will be extremely uncomfortable and there is a high potential for a scene to breakout. My Mom has also made it very clear that she doesn't want to go if her husband isn't invited because she will feel disrespected and that I am not "doing the right thing" by inviting everyone. They have effectively put me in a damned if I do damned if I don't situation. The hardest part is that I totally understand where they are both coming from, it is also not right of them to put me in this situation. My current leaning is that I don't really want my moms husband there, because he is literally only in my life because my mom married him and he should not be the reason anyone goes or doesn't go. My Mom will feel like I'm choosing my dad over her if I decide not to invite her husband, but I feel more like I would be choosing my dad over him (not over her) as she can still come without him. Really he is the common denominator for all of the issues. Yes I wish my dad could get over it and try to ignore the guy, but he won't, the guy is too much of a trigger. And yes I wish my mom could get over it or if her husband was like "ya I get it, you need to be at your sons wedding and I don't want to be the one to cause issues, so I'm cool not going" but he won't because I think he gets off on our family drama. Another thought I have is telling them both they can't come if they either one of them tries to make an ultimatum. IDK, this is a huge pickle I'm in and any advice or thoughts on the matter will be greatly appreciated. Let me know if you need anymore info, and sorry for the long rant. Thanks in advance!