Towns around Lake Como that would be best for someone with mobility issues

Lenno, Menaggio. You can take a water taxi from Lenno out go Villa Balbaniello very close by. The grounds are beautiful with stunning views.

I would say specifically not Varenna. There are loads of stairs, narrow paths, inclined paths. I love it, and my wife loved it by now she is disabled and has trouble with uneven surfaces I don't think we would return at the moment. There is one multi-story carpark near the church and one ZTL area which looks like it would be good for parking but isn't, across the street, and the rest of the parking is along the road into the village. It is beautiful but with mobility issues I wouldn't be sure. Up and down to the train and the ferry is a little steep.

Good luck with finding just the right place that will be just perfect for you all to visit.

Not Varenna and not Bellagio. Lots of stairs in both. However you can visit the front lake in Bellagio, which is flat.
Menaggio is best in your case. You could actually easily stay in Menaggio (I dont care for Como much myself, too big and not a quaint village). Menaggio is flat near the lake. Stay there. Try this hotel:
www.hotelgarnicorona.com

I also encourage you to stay mid-lake rather than in the City of Como. There is a nice medieval quarter there but that end of the lake is less scenic.
Mennagio has a flat landscape, a promenade.

I sent you a private message. Hope you got.