I believe an intervention would work against your interests. H is on his own journey, busy defining himself in opposition to others. I think there are very serious boundary issues between him and his parents. It is because of this that I think having one of his parents in court WITHOUT H having steeled himself for that MAY work in your favor. But, I really think that if his parents stage an intervention, he will do whatever he has to do to get out of that uncomfortable situation and then refocus on defining himself in rebellion against them.
Here is an idea. First, get your legally binding agreement in place. Second, worry about any intervention.
Best case: intervention succeeds, and H turns into magical perfect H and comes home or H turns into magical perfect XH and showers you and the family with extra support
Worst case: intervention fails, H becomes even more bitter and reactive and tries to reduce support, but it is too late as legally binding agreement in place.
Either is better than the worst case if you do the intervention first:
Worst case: intervention fails. H pushes against everything in the settlement agreement. You get not one read sent more than the state forces him to give you.
So, given you are engaged in the inappropriate conversation about ILs interfering in their son's life, ask them to back off until settlement agreement in place.
