I really feel for you MooMoo 22. It reminds me of my MIL and my sister-in-law! They both told me off for feeding on demand, not limiting the time on each breast, not getting into a routine etc. etc. But they lived a long distance away so I could cope with it! Your situation is far worse having MIL on the spot, and she sounds even more interfering than mine.
So much good GN advice above. On the question about which method of getting it over, I wonder if you could convey the message in lots of ways? She obviously has to be handled VERY firmly, however understanding you are about her motives and the autism!
Could you write an email, send a card, write a snail -mail letter, speak on the phone, speak in person at your house and at hers, having a walk together and even having a coffee or lunch together. Any or all of these would be even better if your partner was also saying the same thing. If you make sure the message is the same in all of them, it might get through. If that feels weird, use slightly different words/phrases, but make sure the same message is there every time. Keep it simple so she can't misunderstand, and if it were me, I wouldn't mention her feelings or be understanding of her point of view. I would just say clearly what you want and expect, using the 'broken record' technique if you are talking to her in person.
Very clear consequences; very clear on what is not allowed. I know you have tried some of these approaches before, but I feel a multi approach with the same FIRM message could work as long as you follow up on what you've said will be the consequences.
She seems a bit like a young child that is testing boundaries all the time. It's quite natural for a child to do that, and she may not realise she is doing it, but because you are a kind person who cares about her, she knows what to do to get her way; verbal abuse, tears, shouting, trying to make you feel 'mean' and unreasonable. Using the first precious months of your baby's life, when you are at your most vulnerable, to get her way.