I agree that you have to be symphatethic with your MIL, she is going through a difficult time. What is important is that this time doesn't become a permanent one, hence my suggestion of the ground rules.
When we were living near my MIL (20 mins away), she used to drop by 2-3 times a week before 8:00 am. She rang DH 2-3 times a day and even dared to accuse me of liying about him not being at home, if she didn't find him at his desk (which happened regularly being my DH involved with lab work and teaching). On weekends, she expected us to drive 30 miles to have lunch with them, she started ringing at 7:00, asking us to pick up something from her other house, bring bread, help with something, when are you living?, shouldn't you have left already?, etc. By midday she would have rung at least 6 times.
After we moved away, she kept ringing first because she "didn't authorise" the move, afterwards because SIL was having problems at work, because she was selling the summer house, because FIL got cancer and died(very justifiable at the point, and I paid for DH to fly over every weekend during that time), later because she got another house, because she hated SIL's partner, because she had problems with her neighbours, and then DS was born... ringing every other day at 8 asking us to allow her to see him on the webcam totally ignoring our comments (and needs of silence) of DS going to sleep at 7 or in the process to. And then she decided I was an evil woman blocking her from seeing her grandson... so every time I picked up the phone after 7 it will be her being rude and accusing me of everything she could think of.
A couple of years ago, I stopped answering the phone after 7, and after other family dramas, I asked DH to ring them, whenever he had some time to do it, from the office, and to ask them not to ring our home number (may sound drastic but... by then MIL had even asked DH that he had to choose between me and her, so I was already more than fed up).
We are in the process of getting divorced now, and I really wonder how different things would have been if DH had being more "defensive" of our personal space when all this started. It is not that she is the main cause for it, but this has definitively played a big role in the relationship break up.