Question is IF a pass will even be worthwhile for this. And DO note that in the Swiss about all trains are UN-RES anyway-there is never as such a 100% garantee on seat-if all full, you stand. That is the theory, in practice there will usually be a solution-at least you can chase ignorant pubers away from the seats for seniors-but you can be sent away from these by handicapped.
The ALT means you tie yourself in advance fares-the earleir booked, the lower the price and money gone if not coming. Same-same as airfares on the cheap. Applies to all trips IN FR and IT as such.
But with that pass you also have to make the RES for FR and IT, but as such its easier to change them or as cost is fairly low, even make 2 or 3 if you do as old-style beezness and want to make sure there are more chances. It is also a bit anti-social, as so often for beezness. Expect RES to be only possible from 2 month advance-and there is no real reason to wake up early on that day to go computer-a day or 2 later is also OK, 2 weeks also.
I would suggest you do a thorough search for LOCAL passes in Swiss-that cover that mountain area, Berner Oberland or Roof of Swiss. EUrail covers nothing south of Interlaken, but gives some discount on sky-high fares.
In the Paris area-Transilien is also a quite cheap 7-day ticket /pass for local trains/metro/tram.etc-you need first to have Carte Orange chipcard made and bring a foto for that-and some time.
All large cities will also offer local day or 24-hr tickets aimed at visitors.
MtStM has no station, but in summer SNCF has a special direct bus from a stop nearby at rather high cost-you pay for that. OR search into other possible local cheaper buslines. IF its just a daytime visit, also consider booking locally 1day trip by tourbus.
Also note-to catch out the unaware, that 1/5, the workers celebration day, with red flags is widely taken serious in FR and IT and not at all in CH. It may also mean that a lot of transport does not run then, to allow the jolly workers to join the processions to blame those beeznessmenny for their exploitations.
Here is information about different train passes in Switzerland that could be of interest for you. The Eurail pass is not included in the comparison.
https://olliechinny.com/swiss-travel-pass-2022/