First, hugs to you for having so many things to deal with, and best wishes for your health to improve.
As for your MIL, I may have misunderstood some detail, but I gather that you have asked your husband to speak with her on some points like calling ahead before dropping in to check on you/help, and your mother has spoken with her about giving you space - but you have not spoken directly with MIL?
My suggestion would be that *you* speak with MIL, start by acknowledging the ways that she has been helpful which you very much appreciated (it sounds like ther are at least a few) - then explain to her that while you appreciate the knowledge and care she has and offers, at this point in time you are comfortable with the medical care and advice you are receiving, and just acknowledge that you have periods of time, on treatment days or otherwise, when you just prefer to conserve your energy, be by yourself other than needed medical treatment, and while you appreciate her desire to help and be involved, you find it better for you at this point in time that you have your alone time as *alone* time, and can enjoy her company when it is a planned get together.
If she just really, really wants to check in on you periodically without pre-planning, maybe you can suggest to her that you will use one of those door signs new parents often use hung on the front door that say "baby sleeping" so folks know not to ring the doorbell, tell her you'll use something to signal "drop in visits welcome" or "not".
Off topic, but related, my family including myself are kinda loud, loving Southerners (US), my long term boyfriend's family are stoic, loving New Englander's. We have really different communication, and general comfort zones for some situations - boyfriend's mother would always want to be dressed and prepared a certain way before either visiting us, or having us at her home for a meal. I would generally feel the same for going "out" but at home am more relaxed for an informal family meal. As we live in different states, this was never an issue, or even a distinction I recognized until boyfriend and I rented a condo for a month at the start of the year to be near his mother and enjoy the beach (we work from home enabling short term relocations).
I thought we'd spend many nights having his mother over for dinner, or cooking with/for her at her home. It wasn't until after the second week in the condo when I asked boyfriend if twice a week was enough, that he explained she felt a need to "dress" if we were coming to dinner, or having her to dinner, and twice a week was as often as that felt good for her. This surprised me, in part because of my own practices, but also because we'd spent 10 days at his mother's before going to the condo, dinner together every night and no special need to dress other than casually unless we were going out. Months later, another family member mentioned that his mother had wondered why we rented a condo rather than stay at her house - which we do often, and had just done. I mentioned that I had not wanted to be a guest in her home for six weeks and prevent her having the privacy she is used to for that period of time, but loved being near her and seeing her often.
My takeaway from all this was that communication is so important, because what "feels" best/right can vary, but if different views and preferences are stated and known, everyone can feel the love and limit the consternation.