I fully understnd where you are coming from I really do.
I had (still have) a MIL like this and what with going 18 days over it drove me up the wall.
I would also be wary - because however lovely your MIL normally is and although understandably she is VERY excited - some MILS and mums go a bit lalala when 1st Grandchild is born and kind of forget that they are actually the grandparent and not the mother!!
I could tell a few tales of MILS/Mum/other self important relatives turning up univited at maternity hospitals/units etc - albeit being made to wait in the corridor but then the new mum feeling under pressure to let them in to see the new baby just after - when in reality they wanted to be alone at that time.
My MIL lived 350 miles away but they set off to come and stay with us the day I was discharged from hospital - uninvited in the fact that we had said they could come and stay once we were home and sorted but they took that to mean straight a bloody way!!. They set off at 11am and I was dischared at 4pm. I had 3 hours before they arrived!
Without wishing to worry you - never having had a child before you have no idea how it will go or how yo will feel. I ended up with a C section and just feeling totally overawed by my beautiful new baby. I just didnt want loads of people around me!
Having a baby is private and personal. Yes it is a new grandchild, neice, nephew, cousin etc etc to everyone else but it is also a very personal and special time.
I would for peice of mind just say you will let them know when "it" happens. Just dont explain what you mean by "it". If you don't call her when you go into labour - the chances are once the new baby is here it will all be forgotten anyway and if not you can always make excuses and just say you were worried, it was happening fast etc etc! It would only be someone totally unreasonable who would hold it against you.
I absolutely hated the constant phonecalls and lied blatently to people who insisted on calling several times a day. I didnt tell them I had a date to be induced in the end! They ended up with less info than the chilled out people who just happend to call iykwim!
Also with baby number 2 I added 10 days onto my due date at the very beginning and adjusted it to 10 days after my due date every time scans changed my date. Something to maybe think about if you decide to go back for round 2 another time.
I dont think you are being unresonable at all.
Do what you need to do to make this your special private time - if you dont you will regret it. My DD is 11 and only the other day I had a conversation with my DH about how upset I felt at is parents self inviting themselves etc and was surpised how upset and emotive I still got about it!