This is exactly how I want to go about things! They try to not leave me an opportunity - in this case, after she argued with him, she left for the night. DH came to me and asked if we could stay a few hours later, and after some back and forth, I agreed. I did not find out what his motivation was until later. If his Dad takes issue with something, he'll wait until we leave and call DH on his cell a few days later. That's why I gets so anxious about visits - everything will seem fine until his phone rings several days later. Once his Dad even drove up here, chatted with us both, waited until I'd left the room, then took DH outside to express some complaint he had. They go out out of their way to avoid me! How do I get around that?
Like others have said, this needs to be addressed directly with your husband. There's not really anything you can do to get around the devious and manipulative way your in-laws are behaving, and I wouldn't bother wasting my time trying to reason with them until there are some firmly unmovable boundaries set up. This has to be done by your husband though, not you, since the person they are manipulating is him (because they know they have no control over you). If your husband is "scared" to lose his original family because he's not doing exactly what they want the way they want him to do it, then that is a major stumbling block to a successful marriage. I'm curious, did you ask him how "scared" he was of losing his wife and child?
Sounds like it's time for some counseling. I'm sure a secular counselor will help a great deal, but also, don't ignore any counseling offered by whatever spiritual community you might be a part of, if you're part of one, that is. It's often free, and they are very likely to support the message that people in a successful marriage can only be successful if they put the new family they've created first on the priority list, while still showing respect for the original families both spouses came from, of course. Any older couples you know with successful marriages might be able to help you as well.
Good luck on dealing with this. I'm dealing with this on a minute level in comparison to you, but fortunately, my DP completely ignores the most destructive stuff that comes from the most destructive person in his family. Plus, both our families are over 3000 miles away. Which helps beyond belief!
Wait...can you move to, like, Hawaii, or China or something?