Senior with limited mobility would like to see the natural beauty of quintessential Switzerland.

Have you considered any train excursions in Switzerland like the Bernina, Glacier or William Tell Express.?? If your mom can make it up the train steps, she will be rewarded with breathtaking views from her seat on the train. Sounds so easy peasy to me I'm planning this for my 2018 trip.

I am sending you a PM.

If she is averse to heights then the steep cog trains and telepheriques will probably not be to her liking, right?
If so then the previous advice would be the least stressful, not to mention the most luxurious. ;-)

Most of the "high" mountain resorts are at about 5000 feet. That would include Muerren and Wengen. Nothing higher for the most part. Muerren might entail a bit of walking, but Wengen has hotels right at the train station.

As your Mom can't walk far and is somewhat afraid of heights, the higher mountain towns such as Mürren probably wouldn't be a good idea, as a short cable car ride will be necessary.

Lauterbrunnen is very accessible by train from Interlaken Ost (about 20 minutes), and it's a very scenic ride. However, some walking will be required from the station to your hotel (depending on which hotel you choose). If you'd prefer a "higher" location in the mountains for the views, you could consider Wengen which is accessible via a short train ride on the Wengernalpbahn from Lauterbrunnen station (it's a cogwheel rail that climbs up the side of the hill, but no cable cars to deal with). Wengen tends to be a bit more on the "posh" side, so you might pay slightly more for a hotel there.

I have a general suggestion: Look at the postbus routes. Postbuses are smallish buses that mostly run up valleys not served by trains. The ones I've taken depart right outside the train station. You can make out-and-back trips through very scenic countryside, planning a meal break and quick walk-around at the end of the ride. There used to be at least one or two special, tourist-oriented postbus routes that crossed multiple high passes. I don't know the altitude of those; they might be too high for your mom.

Postbus routes are shown on the Swiss transit map (should be easy to find online), and the various Swiss travel passes probably convey some sort of discount on the buses. Most likely the Swiss National Tourist Office highlights some of the routes on its website.

Sorry not to provide direct links for you, but I'm traveling myself at the moment and haven't figured out copy-and-paste on my tablet.

Actually, she could reach Murren by cog train instead of gondola. She wouldn't notice the rise in elevation riding along in a train the way she would in a gondola. The town is long and narrow along the side of the mountain. There are hotels on the side of the town near the cog train. I wouldn't rule out Murren.

Edit: I forgot about the initial gondola. What a shame to miss Murren due to a short ride. Stand in the middle and close eyes.

Although you will need to take a short cable gondola ride from Lauterbrunnen station to Gruetschalp to get to the other train. And it can be a bit of a walk to central Muerren, although it is fairly level. That is why I suggest Wengen. You can also ride the train from Wengen up to Kleine Scheidegg at the base of the Jungfrau. Great views on a nice day and little walking.

Alas the old funicular up from Lauterbrunnen to Grütschalp (connecting with what Rick used to call the Panorama Fahrt) is no more. Several years ago the moving mountainside finally proved too much for it, a relatively short cable car was built to replace it, and the old track lifted.

As a matter of trivia, for the seriously trivia mad only, for people who travel on staff passes like I do (because of working on the British railways) that cable car is still considered a train and included unlike most of its neighbors. OK, unglaze the eyes and we will push on.

That cable car is fairly vertical, but exceedingly smooth and without the sharp intake of breath occasioned by the Gimmelwald-Stechelberg cable car. It is so large that I have known people with height worries to stand in the middle surrounded by friends and others and not be bothered at all. I personally stand in the rear right corner looking back down hill so I get the best views.

Would your mother have any issues? Dunno. It is a little steep for a short while after leaving the cable car (no steps required if you go out the back of the Mürren station and around) before you get the most scenic part of town.

My mother and I thank you very much. Seriously we appreciate the time and effort all of you have taken so that we can have a great trip.

THANK YOU!!!

Engelberg, a great valley with a cheese making monestary, a great doctor, and beautiful mountains.

Saas Fee, Zermatt's nicer, more fun cousin--albeit without the Matterhorn (OR the huge, shoulder to shoulder crowds either).

Binn, or somewhere in the Binntal (Binn Valley). Some quaint little houses with old, stone bridges and several very tasted restaurants.

Luzern. You could go from town to town via the lake boats and spend every other day in a new place.

Good trip!