First off, let me say that I sympathise with you regarding the step-parenting/ ex-partner relations stuff. It's not something I've ever dealt with myself, but I can imagine how difficult it is.
I think there's some good advice throughout the thread on how to deal with your husband, his ex, and the custody arrangements. But I wanted to take a minute to throw in my two cents, from a divorced kid's perspective.
My parents split up the day I turned 18. Like, literally. Because I was 18, I didn't figure into the settlement at all, and as my sister was 16, the arrangement was 'she can come & go as she likes' (a situation which, like most teenagers, she exploited shamelessly). There was never any discussion about holidays. The stated assumption was that we'd simply make our own arrangements. The unspoken assumption, on everyone's part, was that we'd each get to do things exactly the way we wanted. See the problem yet?
Those first several years were hellish! Sis & I were just trying to carry on as normally as possible, doing all the things we'd always done. Dad was trying to let us, but was also trying to work in stuff with his new family, and wanted to include us. And Mom was a never-ending source of tears & guilt-trips, because we were choosing to 'abandon' her. It was a horrible position to be put in, feeling like we were solely responsible for our parents' happiness. I'd have given anything to be able to say 'sorry, but the judge said so.'
So, maybe you can explain that to your husband, to show why it's absolutely vital that they work out some kind of specific, legal custody & visitation arrangement, especially for holidays.