Hi there, Sashabreeze! I have 11 week old twins and a 4yo DS, and am right in the thick of grandparent help. Thought I'd tell you a little about how it is going.
I can so relate to the relationship you describe with your MIL. Mine too adores the kids, passive-aggressively criticizes me, had an only child but wanted more, thinks I'm insane about food and my preference for nontoxic products in general ("you think everything is poison, haha" says FIL). Her main mode of communication with me is offering a nonstop stream of unsolicited advice. I don't at all think she is a terrible person. In fact she is so giving and thoughtful, but we do clash in many ways. Not someone I would otherwise choose to spend so much time with, but ... I love DH and she is his mother.
My folks and my in-laws are taking turns renting a studio apartment around the corner from our house, from March through the end of September! I am so grateful for the help, and wow how generous is that to offer to rent a place and help out so long for free. And yet ... it is so challenging to find that balance getting the help we need and getting the space and privacy we need. I am not crazy about the way she interacts with DS1, either. Nothing terrible, but not our style of teaching or discipline. I am struggling with wanting our kids to be raised how we want them to be raised, but letting the grandparents have their own relationships with the kids.
It was hard to know what to ask for while I was pregnant with the twins, because I didn't know what to expect. They were born at week 35, and I needed a LOT of help in the early weeks at home, after a few weeks in the NICU. Nursing is going fine, but nursing two means having both hands full much of the day - so different than walking around nursing a singleton in a sling! I've been making adjustments (so so carefully - trying to avoid big conflicts) to the arrangement as time goes by, trying to minimize time together and to clearly define roles. (For me, it is different but nearly as difficult with my own mom.) So hard to be clear about what we need without insulting anyone!
I just recently started writing out the days' schedules and the menu plans week by week. I am not usually such a planner, but I realized I need to take charge since this is going to last for so long. I only just started doing this, so we'll see if it works! I'm hoping that if it's all written out, it will require much less explaining and negotiating each day. I found that when I was in survival mode during the earliest weeks, the grandmas naturally "took charge," which I guess we needed for a while, but wow did it drive me INSANE! I definitely need to surrender some control while they are helping, but hopefully just for a couple of hours a day.
So, I'm now looking at what is in store for each day, and asking for help ONLY for the times when it makes a big difference to have an extra set of hands. Otherwise, I'll do it myself, thank you very much! Maybe your planning skills will serve you well. I've had to work on cultivating mine!