You did not give the month of your 2020 trip. Aside from being busy all summer, there is also the anniversary month, when things are especially tight. Just summer crowds meant that some of our parking spaces (rental car) required 1/4 mile walk to the actual site. There are also many sights (like fields with gun emplacements rusting away) that require a lot of level walking. I personally didn't care for the skilled specialist guide tours I heard around me at the beaches - too passionate and authoritative, but they do have reams of laminated enrichment material to illustrate their tours. I don't doubt that you feel like you got your money's worth, however.
My 93-year old mother is a poor comparison, but you need to consider whether your mother-in-law might not have a sore knee or a twisted ankle 18 months from now. You have to make commitments far in advance, and Travel Insurance gets expensive after 62 or so. I'm not a car lover, but I was happy to have a car in Bayeux - we chose to stay outside town because we had the car.
Mont St. Michel, despite its towering position in tourism, is not served by a dedicated rail station across the street. It's a multi-seat ride by public transportation. For a 73-year old, I would want a private car so I could drop her at the shuttle bus stop, which takes her to the electric shuttle, which takes her to the fortress. (That's not as bad as it sounds. But when you add age and heat, it's not as trivial as the actual 1/2 mile (??) journey from the farthest parking lot.)
I'm not trying to talk you out of this. I think a 73-year old can do this itinerary. It's the reliance on public transportation that is too optimistic.
