I hear you. Her plan is not foolproof, anyway. The problem with her preparing a separate plain turkey breast/chicken/whatever, is that the odds are extremely high that somewhere along the line, she, (or someone else), will mistakenly touch the "safe" version after handling the dressing, (or a contaminated part of the turkey), without even thinking about what they're doing, and you will be "zapped" without them even realizing what they have done.
Your MIL is not unique - that attitude exemplifies the general attitude of most family members about relatives who have this disease. It's just a little diarrhea, so what's the big deal? They assume that Thanksgiving dinner is their project, and if they're going to go to all the trouble and expense of feeding everyone, then it is their right to set the rules, (and it is, of course). Also, from their viewpoint, why should they spoil everyone's fun, for the sake of one in-law?
I used to have a lot of arguments because of holiday meals, but I simply told everyone that I couldn't eat that stuff, and I either didn't go, or I went and didn't eat. Eventually they get used to it, (but they continue to offer me food that I can't eat - I guess they'll never really learn), but at least now they don't get upset about it, (well, not much, anyway). It's much simpler if I don't go, of course, because then I don't have to go through another round of arguments at mealtime.

Of course, I have a big advantage over you, because I don't have a spouse and kids who would be more than a little disappointed at not being able to participate.
It's a tough exercise in diplomacy, because you're dealing with people who are dedicated to keeping a long-standing tradition alive, and keeping the family together, and all that good stuff, but unfortunately, they really, really don't understand our situation, because it's impossible for them to picture themselves in our shoes. Subconsciously, I suppose, they feel that such "minor" issues should be put on hold, or ignored, during the holidays, because holiday festivities, and visiting with family, should take precedence over such "trivial matters".
Tex