To answer your question, yes, I travel w/groups of strangers when traveling w/tour groups. Sometimes a friend will go along, but that doesn't always happen and within a day or so, you make new friends.
I hate to say this, but your MIL is behind the chores article as well as the anger your h was exhibiting. She's pulling out all of the stops to get her grandson over there. She doesn't want to admit that she is the reason that you had the email discussion w/her about his haircut. She is trying to not only control, but manipulate your h into brow beating you into your son going to Granny's.
I don't think your h is the one coming up with the chores. He was content to have your son over there in the beginning and now he's pushing chores and the games/shows your son watches. I hate to say this, but you most likely will need to have a serious sit down, face to face conversation w/your h about his anger, expectations and yes, his mother. This is going to continue to escalate and create some serious friction for all of you if you don't nip it in the bud and soon. There are some instances whereby you can't be super friendly when trying to get your point across and she's probably still miffed about the haircut incident as well as the request for the key. You are your son's mother and she needs to respect your decisions, as well as your son's if he doesn't want to go over there.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.