It sounds like you're doing the best you know how with explaining your condition and that you're truly not trying to be mean to them when you get depressed and go into withdraw-mode. They're seeing your kids again and I'm sure that means a lot to them. It's too bad that your husband is having a hard time reuniting. There may be other issues preventing that bridge from being rebuilt...I don't know, past cases where he was hurt by them or maybe just some pride in admitting some wrongdoing on his part as well...not saying that he did or didn't, just speculating. In-law situations are always complicated. I know my dad's mom and my mom have never got along and my dad has problems with his mom also.
Have you explained to your husband all of what you wrote down here? Have you told him that it is essential that the in-laws understand that your past and future behavior has nothing to do with them? That you'd really like to see things healed? (the tone of these questions is curiosity, so I hope you don't think I'm being mean, just trying to understand to be of some help). Does he know that you'd like his support on this issue and if he refuses to talk to them it just plays into their view that you're a *****?
Maybe some people will never understand bipolar disorder or think it's an excuse for acting bizarre. My view is, that's up to them. If they don't want to get it, they won't. It hurts sometimes, but if it comes to that, you need to release yourself of guilt since you're doing all you can to explain your condition and that you aren't trying to hurt anyone. So, forgive yourself. Do what you can to maintain stability and ask for help when you need it.

I think you're doing pretty good on that front.