My MIL knows no boundaries but even she wouldn't insist on overnights.... Forget about them being alone... And EBF to top it all! Even I know that WHO says they're supposed to be EBF for six months (I'm bottle feeding only but that's here nor there). She shouldn't insist upon it.
Just why does she want a stressed, screaming newborn at 10,12,2,4,6 and 8 anyway? (You know they'll wake up all the time if they're unsettled)
Her job as Grandma is to have occasional fun with the baby. Not slog through all the mucky parenting bits. She's done that already.
Stop being polite in your refusals. You've done enough politeness and explaining. She's not returning the favor and it's time to use her tactics against her.
Say one word: No.
If she argues, mirror that back on her. "You're not letting me be a grandma"? Well, you're not letting me be a mother.
And her not treating your other children well is horrible. They're perfectly innocent.
It's the only way I can get through to my MIL. Being polite turns me into her doormat, and believe me she expects a red carpet. Sounds like yours expects that plus the paparazzi.