TealDragon, I am so sorry all this is happening to you, and will send you and your horse all the virtual support I can. A close friend told me that the hardest part of dealing with this kind of major health issue was that other people either wanted to hear all the gory details all the time, with an avid and often quite offensive interest, or all they wanted to hear was "Fine" and expected her to take care of them emotionally. People also did not understand how utterly sick and exhausted she was, all the time, no matter how often she told them, and kept wanting her to do things and more or less entertain them when she could barely move and was almost always sick to her stomach. She DID recover, and you will, too.
I think what might work is telling FMIL that you really could use her help if she would come over on Specified Day and clean the house, maybe when DF is home to help her, do the laundry and cook for the week so you can rest. When she comes, go into your room and shut the door, and tell her you will come out when you are rested, thank her generously for her help, and put on headphones. LOCK the door, too. I would also tell her, AND have DF tell her, very specifically, that you point-blank do not want to discuss your illness or treatments because doing so makes you sick to your stomach. SOME people may like talking about it all the time, but YOU DO NOT. Wording has been posted by several PPs. Be firm, and divert sharply.
The rest of the time let the phones go to voice mail and do not answer the door. Have DF put that film on the windows for both of you. You are probably too exhausted to even contemplate doing it yourself. You CAN have DF tell her you need to sleep almost all the time you are not at work or being treated, and that you simply must do the above, except on Specified Day. Interaction exhausts you.
Do what you need to do to get through this. Be kind and gentle with yourself, and ruthless with others. You will get through it, and much better, this way. And there is nothing rude about it.