I think you are going to have to jog along with a bit of this and a bit of that, as a family, rather than my-rules-go-regardless-of-the-needs-of-other-people or I-must-defer-to-Grandma-so-as-not-to-upset-her. The way to success is likely to lie somewhere between those extremes. Sometimes the baby's needs are more important, sometimes the mother's, sometimes the extended family's.
I would not have taken MILs advice on weaning had she offered it; otoh when FIL had a heart attack in our front room, obviously his needs came a long way before my baby's routine.
When 4 different families with children at different ages were staying with my parents in the summer, I thought it was perfectly reasonable for them to say: THESE are the mealtimes, rather than to kill themselves trying to fit around 7 different children, all with different routines. And I did feel it was very beneficial for my children to get used to fitting in with others.
But that didn't mean my parents had a say in those parts of my parenting that they did not have to cater for. I have been making polite noises for the last 5 years to my mother's hints that I owe it to my dcs to throw out the fish tanks (my great interest) to make room for the piano, because children without a piano will be at such a disadvantage. That, I think, is definitely a case of my house, my rules. But if a grandparent chooses to use a different nickname to the one I wanted- imho life is too short to argue. And who's to say they won't grow up with fond memories of her nickname and a grounded hatred against the beautiful name I gave them . Though I did put my foot down at Borg. Honestly.
I have never found it does my dcs any harm to be a little spoilt by grandparents at that house: I have complete confidence in my ability to discipline them when I need to.