Hi, Margo. Yes, people very often show up with meat and say "We brought DINNER," as though the Professor and I hadn't been cooking all day. We are good cooks, too! It happens often enough that we have the separate outdoor cooking area, etc. I have never been anything but a veg, though the Prof became one in college, before we met, and the cooking smells squick us out (sorry, but it is true.) I would blast them to e-hell and back, but the Professor usually handles it, and more gently (I do love that man!), and it rarely happens twice, except with a very few people who do it specifically to be offensive, and whom we can't entirely avoid. Which is why there is the faculty club, and there are also restaurants. We do not invite them to our home again after a second violation. These kind of people have been known to invite us and make sure there is absolutely nothing either of us can eat, not even the nibbles. Yes, this stuff really happens, and I do not know why. This is not making reasonable accommodation for guests, which is the etiquette issue here, it is setting out to be deliberately offensive to one's hosts or one's guests, which is beyond rude, and a power play. We avoid such people whenever humanly possible.
The grace thing happens at other people's houses, mostly. We have an invocation of gratitude for the life forms that make it possible for us to live and practice Buddhism -- there is no easier way to say it without getting into religion specifically, and we are not religiously obligate vegetarians -- but we do not do it out loud but rather quickly, personally and silently when we have non-OurSect guests or in other people's houses or restaurants. And, yes, quite often I find my hands being grabbed. I do not begrudge others their beliefs, certainly, and if they do not know us that well, I presume they are simply making "interesting assumptions," but it is hard to retrieve my hands without calling more attention to the issue than I would like. We duck out if we can, of course, if it is not a sit-down dinner, though people may wonder what we are BOTH doing in the bathroom (evil grin). If someone says, at our house, "Aren't you going to say grace," or something similar like, "I'd like to say grace," the Professor, as host, takes over and performs our invocation. Which does not involve holding hands. <G>
As far as the art thing goes, I stick to my guns. Display what you like, and so will we. Even if it IS that three foot all p*nis or a Buddhist version of the Mapplethorpe print of Jesus which offends so many people. Don't like it, don't look. In your own home, you have the right to display the art that pleases you. I don't have to look (I don't like the Mapplethorpe, personally, either, though it isn't religiously offensive to me). But there we, as guests, have a choice -- to look or look away. And so does the OP's MIL.