My 27 cents. And these are not my decisions, so they are worth even less. Stairs can always be added. They leave something for the next guy to do. They are buying the dream, you would just be selling a house.
A home elevator could be just the ticket at that time. More and more popular as we keep living longer. To the future!
Anyhow, wife and I put an addition onto our 40 year old tri-level two years ago. Not the kind of home to get old in, for all the reasons others have mentioned. One feature of our too narrow stairwells (mjncad -- you got the 4' width right): if stairs are walled in you can place rails and have a FULL wall surface to grab/fall against. It has proven useful when I slip (and no, I am far from old).
One decision we made about toilets: ADA compliant toilets work great if you have long calves (what ever you call your lower leg section), but they are not good for that 'optimal evacuation position'. That's why as crude as it looks, using a hole in the floor and a full squat is better for you. What we plan to do as we rehab the existing bathroom and powder room: The main bath will get an ADA toilet. The other two are going to be low height, short bowl models (space is a powder room issue). Even good toilets are only ~$250 at Lowes-Depot. Easy to change if a swap is made later. Let Mom use them both and you as the likely prime visitor (your daughters will likely like the smaller one best anyhow).
Lever door latches/locks. We put outdoor grade levers on our house. I bought self rekeyable units. Found an on line place that with the order I had was enough to get free shipping. Most major brands have them. I had them send me two sets of keys: one set I used for the outside doors, one set for the interior doors (I keyed all doors to a room). I also made all room doors (save the closets) solid core and mounted them with screws through the jambs. Too easy to do for security sake. Place the core cordless phone (the base unit) in a room that is most likely secure. Put the satellite units in each room that can be safed. Just a thought for Mom.
Wire rooms with light switches near where Mom might be entering a room (or, where her bed might be). Not much money, and seemingly too convenient, but this is for making sure she has light when she needs (versus wants) it. I am a big fan of overhead room lighting. Wife didn't like it, as she still puts lamps up, but I have can lights and decent (sometimes even good) lighting in each room. Vision is a big contributor to falls, as much as snagging carpets or tripping over the dog/cat. And one of the biggest issues is that folks get cheap as they age -- 'I dont need more light'. Macular degeneration and cataracts also affect your vision. FYI, follow the manufacturers recommendations for can light spacing. I found this out the hard way. Cheap and 'it doesn't look right' don't count.
We put in a floating cork floor on our concrete. Works elsewhere too. Very easy to install (snaps together, 1' x 3'). Just an alternative flooring. Quiet, neutral temperature, easy to clean, attractive. Not cheap for the flooring, but no finishing and cheap install (overall it's mid-priced).
Always good to see another build with alternative processes. This looks a lot easier than the styrofoam block builds. I wonder what the price differential would be. Something to put under my hat for any possible build on our part. Would work well with SIP I think. And metal roofing (must be getting old, I keep thinking about what is easy to build & cheap to maintain). Oh, probably because I want to work in my shop more! Or get seat time....
I almost forgot: the best part of your Mom's house is it's all on one floor. That is a dream for your Mom, and will work in your favor at resale time. My wife and I think that people will get far more practical in there home sizes.