Christmas itinerary in South Tyrol (Vipiteno/Merano/Bolzano) and Switzerland

Only two thoughts: 1) Moving 6 times in 17 days would not be relaxing for us, and it takes a lot of time away from other activities. 2) Merano is 45 minutes by train from Bolzano - but Ortisei is an hour by bus beyond Bolzano. That would be a long day trip and the days are short.

Innsbruck has a beautiful Xmas market. I would spend the second day visiting the city (for example, the Hofburg palace, almost as beautiful as the Vienna one but much smaller), IMHO Seefeld does not add much.
There is more to see in Innsbruck than in Vipiteno, but you could take advantage of the direct railway link and visit Brixen from Vipiteno. Of course, Dec. 25th and 26th are almost lost days as everything will be closed (reserve restaurant), but maybe the 25th December High Mass at 10am in Brixen Cathedral may be something to attend. I would expect full choir and orchestra for that service.
Merano is not a base for Ortisei. Almost two hours travel
Be sure to understand how you get from Merano to Sils Maria; this involves multiple switching between buses and trains.

As a general rule, consider that temperatures will be on low side, so do not plan too many outdoor activities and be ready to substitute indoor ones if necessary. This is why Innsbruck is better than Seefeld, and this is why I am perplexed about the multiple switching between train and buses for getting to Sils-Maria. This also considering your old mother.

What a great trip to take with your mother! Sterzing is quite small so you may want to look at staying 3 nights in Brixen and visit the Sterzing market while enroute after departing Innsbruck. From Brixen, you can add a visit to Ortisei (40 minutes). While there, consider booking a sleigh ride across Alpe di Siusi, Europe's largest plateau: https://throneandvine.com/alpe-di-siusi-hiking-guide/. It is surrounded by some of the most iconic peaks in the Dolomites and would be a lovely experience with your mother. Have a fanatastic time in the region!

IMHO all these Xmas markets are best visited being based in some hotel and not en route to another destination, as railway stations are fairly small and do not have a baggage deposit. Maybe in Brixen you can find a service, but I am afraid not in Vipiteno. And do not assume taxis are easily available but in the larger places like Innsbruck, Bolzano or Merano.

Thank you everyone! I have decided to skip Italy and shift our dates to stay within Switzerland. We will start on December 31 in Lake Lucern (2 nts), train to Zermatt (2nts) the take the glacier express to Engadin (4 nts) and finish in Wengen/Jungfrau region (4 nts) before flying home from Zurich.

This itinerary allows us to see a little of Italy by doing the funi to Corviglia and a day trip to Tirano on the Bernina Express- both from St Moritz.

This itinerary reduces our hotel movement from 6 to 4 and makes more sense since so many things are closed during the holiday season. In the end, it became hard to find the value in paying more only to see a few Christmas markets. Better to pay less and have the full experience, not to mention many beautiful train rides along the way.