Having taken two cruises on AMA Waterways (Tulip Time and Rhine Christmas Markets,) and on Uniworld (Seine River Paris to the coast and Bordeaux):
For a Seine River cruise, look into Uniworld. Their ship Joie de Vivre is spectacularly beautiful and elegant. I took my spouse, daughter, brother and sister in law on the Uniworld Seine River cruise and they all loved it.
Embarked at the Quai de Grenelle in Paris, with lovely view of Tour Eiffel. Excellent on board experience and lovely fellow travellers made for fun in the evenings! At Honfleur, walked short distance from ship to the town center, looked around, and had a lovely galette lunch in a little hole in the wall restaurant. One of the favorite ports. Giverny in full bloom mid October, another favorite stop. And a full day to the D-Day beaches and American cemetery.
AMA Waterways also lovely cruise, excellent cruise director and much merriment on board due to the season. Ship not quite as elegant as the Uniworld Joie de Vivre and AMA is not "all inclusive," which means some drink tabs, tipping, etc. Not a deal breaker IMO.
If your mom needs an elevator, explore which ships have those. Joie de Vivre did have an elevator and a nice self laundry as well, if you want to use it.
River cruisers do tie up next to each other at ports. You usually can walk through the reception areas from boat to boat. But not always and you may be climbing to the sun deck and down more stairs as you traverse the ships to shore. You may find your balcony cabin is looking right into another cruiser's cabin window. We've had both upper tier balcony cabins and lower deck cabins with just port hole windows. I actually preferred the privacy of the lower deck cabins, which tend to be smaller but full of comforts. Although you trade some light for that privacy.
We are now embarking on our first Scenic cruise (Danube) which I picked for the specific itinerary/stops. We've travelled along the Danube by automobile in the past, and I wanted my daughter to be able to visit Nuremberg (our family comes from there.) The logistics and communications have been a little challenged. If you use a travel agent rather than booking directly with Scenic, they will communicate and send materials to the TA, not to you directly. We got the Glen Moroney letter a few weeks ago, to my personal email. Will roll with the punches and hope for the best, so jury is out on Scenic at this time.