I think you are being U with regard to your MIL. Runs to the doctors, dog walking etc are all things that it is reasonable to ask your DH to do for her.
Regarding WfH, its your DH who is in the wrong here. It is up to him to manage his working time and make himself unavailable during his working hours. I work from home 2-3 days a week, and I'm very clear to everyone - family, friends etc that between the hours of 9 and 5 I am working and unavailable, just as I would be if I was in the office. I also stick to that with my own appointments etc.
So if your MIL wants a visit or the dog walked, it needs to be outside of DH's working hours. He's saving his commute time, so should have some extra time to give to her on those WfH days.
If she needs help with Drs appointments, your DH needs to explain to her that she needs to consult with him before booking, and to pick a time when he can help - if she books it for 11 am for example, he needs to say to her "Sorry Mum, no can do, I'm working til 5". It's pretty simple.
Overall, though, your DH is not your possession, and him spending time with his mum and helping out is a good thing, and I think you are unreasonable to resent that.