I don't post often to other's threads (my bad) but I do follow a lot of folks here, including you.
I wanted to chime in on in-laws. I'm don't recall exactly what the dynamics were prior to h's MLC, but it sounds like your MIL was difficult before all this happened (she actually sounds a little like my own mother in some ways). Throw in the MLC and things can get even more dicey.
I think that when this kind of stuff happens, the in-laws (as well as your own family) really don't know how to act or respond. H has changed, you have changed, the life you all shared has changed and they are clueless as to what happened. It must all be so confusing to them.
I think you are right to lay down some boundaries with them. Life as they knew it is not the same and the boundaries are different now. They just need to learn what those new boundaries are. Kudos to you for putting forth the effort to teach them. They are, after all, your S8s grandparents, so some kind of mutually acceptable R has to be considered, for S8s sake. They need to learn and, unfortunately, it seems that it has fallen on your shoulders to do it. It's a tough wire to walk, but I think the effort put forth now will make life much easier for you and S8 in the long run.
I also wanted to add that traveling without a companion is not all that bad. I do it often and actually kind of enjoy it. I don't have to accommodate anyone else's ideas of what would be fun and can avoid doing things that really don't interest me. I always plan what I do around what would be enjoyable to do by myself and, surprisingly, I've met some very interesting people. Another thing I have discovered is that once hotel people realize you are on your own, they go over the top to help you out and "cater" to you. I may look like some sad Miss Lonely Hearts, but I get to take full advantage of those who "pity" me. Sad, but it seems to be true, and I've learned to use it.
My best to you and your S8.
Me: 59 and holding H: :53 Me: 1 S, 1 D, both grown M: 19 T: 23 BD: 9-23-2013