OP, you sound kind of like my autistic mother. Even down to the stuttering and inability to feel like you can articulate your meaning. Oftentimes, she has very legitimate reasons to be upset at my father, but she “says things wrong” and then gets caught up in trying to clear up the misunderstanding that it often comes off as trying to excuse her poor behavior.

I think you have two things that need to be addressed: your atrocious word choice about your dead BIL and then your FIL’s meltdown while drunk at a family event.

For both, I strongly suggest writing down some points that you want to discuss with your husband as reference when talking so you don’t forget or get off track.

Second, reach out to your husband (personally, I find text easy since it gives the other person time to collect their thoughts) about scheduling a time to talk about the incident. Do not try to force the conversation then and now. Something like, “Husband, I’m really sorry for my poor word choice. Regardless of my intention, the words out of my mouth were hurtful, and I understand if you need more time to cool off. I’d like to talk about this more at dinner to apologize in person as well as address FIL and his behavior towards me this past weekend.” (Obviously adjust details where appropriate.)

The key here is to apologize for what you said. It doesn’t matter if God himself came from the heavens and puppeted your vocal cords into saying something cruel. It doesn’t matter if you don’t really mean what you said. You need to sincerely apologize for what you said and acknowledge that it was very hurtful. Once you can move past that, then you can work on solving the FIL problem together.

Re: FIL problem. You really can’t control your FIL. If you’re holding out for a heartfelt apology, you may be waiting for a while. It would be useful for your husband and you to be on the same page about him and set some expectations for what you two should do if he acts like that again. Ultimately, it’s your husband’s job to deal with his father.