Why are you so keen for her not to pick your ds up from school?? School staff are usually good at being tactful - if she is being a bit interfering you can reiterate to his teacher that you only want them to communicate with you or his dad and are happy for them to tell his grandma this if she tries to raise issues with them herself.
I wouldn't commit to your weekday that you have off work as you may start to resent having to go to your mum's for tea every monday on what should be a day off where you can pick your ds up from school and spend a bit of time with him.
I wouldn't commit to the Friday either as, as others have said Friday is often a time for playdates/after school parties. Plus he will be coming home with homework and stuff that needs doing. Also if your mum is a bone of contention in your household this will set the tone for the weekend won't it.
I would suggest tuesday and/or wednesday she picks him up and does him his tea, and then he's home by bath time (you pick him up on your way home from work, quickly stop off, he's ready to go, and off you go home).
Have you thought about what you will be doing in the school hols?
My mum lives less than 2 miles from me, DS and I see her every 2 or 3 weeks, for a few hours. My other sis usually will pop in at some point for a few hours on a friday or saturday, my youngest sister and her kids see my mum even less than me. The point is it shouldn't be such a big deal, and it shouldn't be so rigid and set in stone, and need planning this many months in advance!