Speaking from mobility-challenged experience - another vote for St Germaine. On our last trip (2019) I was severely challenged with a foot that needed some corrective surgery. The sight of steep metro stairs made me queasy and cobblestones were a total no-go. We were staying up a hill (ok, that was a mistake) near the Pantheon, but I managed by using the Citymapper app to carefully map out my route options to minimize walking and relied heavily on the convenient, excellent buses. Riding the buses daily to and from the 5th arrondissement, as I looked out the windows at the flat sidewalks in the leafy, St Germaine area, I kept asking myself, why am I not staying here!?!
In two weeks I’ll be in Paris again (yay!) staying in… St-Germaine-des-Prés. Although I’ve had my surgery and recovered, I’m still needing to be cautious (cobblestones!) and will pay special attention to mobility issues while there and I’ll report back in the review section.
That said, I think that many of the other areas mentioned could work with some planning. I’m a bit of an arrondissement-hopper, always looking for my next favorite neighborhood, so we’ve stayed in several of the areas mentioned in this thread including rue Cler, le Marais, and the Palais Royal area (not far from Les Halles).
The Marais would be my next choice especially if the hotel is within a block or two of rue Saint-Antoine or rue de Rivoli - where the 69 bus runs - and away from the heart of the district where sidewalks are slivers and cobblestones reign. The aforementioned Hotel Caron Beaumarchais fits this criteria.
I do appreciate the charms of rue Cler. It is cozy and it does have the Eiffel Tower! However, it’s not high on this list, mostly because transit connections thin out in that area.
Have a wonderful time with your mom!