I'm a mother-in-law, and not all of us are a giant PITA.

If your MIL is elderly and set in her ways, you might want to just not engage her in arguments about Medicare or whatever. Is it really important if you are right or not? She wants to be right, humor her a little. One of you has to be an adult about it all--you will take more pride in yourself if it's you. Right now it seems like a month is a looooong time, but it will be over before you know it, and then you can be free to do and feel what you want.

If she's younger and able-minded, I'd try to come to some kind of terms with her. I hate when these kinds of things give all MIL's a bad name. Maybe she's just feeling misunderstood (certainly, you are!). It sounds to me like the two of you should come to some kind of agreement on something--anything. Try to figure out what that is that you can give over to her, completely, with no reservations. Let her be right. Does it really matter if the turkey is upside down or not? Give in on something and maybe she will be more likely to give in on something else. It just sounds like a power struggle to me. If you don't push back, she'll just fall on her face.

I know it must feel horrible to know she's sitting there watching you operate in the kitchen and just waiting to pounce on whatever you do that's different from her. You can avoid this situation with a little advance planning. Go and do the things you would do normally in those three extra hours, and like others have said, add on some more--especially something you've been putting off, like cleaning out the closet or whatever. Don't just sit there with her.

And, I think I'd strongly encourage DH to take a few hours off early once or twice a week until the visit is over. It's not fair of him to leave it all on you.

One last thing--take notes, in case you're a MIL one day.
Karen