Long post below!
Only you can make calculations using the railway companies websites (SNCF, SBB) and decide what's best. But we can help a bit.
Eurail is generally great value in Switzerland. Trains are frequent, the rail network is extensive, tickets are expensive. You get some discount on mountain railways/cable cars while boats on Lake Thun and Brienz (a fun experience, less busy in the late afternoon btw!) are fully included.
You do need to pay extra for French TGVs (high-speed trains). More info below (very useful website) →
https://www.seat61.com/how-to-use-a-eurail-pass.htm
https://www.seat61.com/interrail-and-eurail-reservations.htm
The Swiss Travel Pass is too expensive for most trips but it is undeniably convenient. Simply hop on hop off and show the ticket whenever asked.
A middle ground might be the Swiss Half-fare card + half-fare discounted tickets. But it requires a little bit of planning.
I'd advise to pick a base (max. 2) in Switzerland and stay there for the whole duration. No faff with luggage every day. It's way better to travel slightly more every day with a light backpack than carrying large suitcases, wondering about lockers, waiting eagerly to check-in, etc.
Some villages/towns you mention are rather small. The main sight are the mountains around, where you can hike or ski depending on the season!
Of course you could spend some time in Geneva or Zurich but really you visit Switzerland for its Alpine scenery, not for the cities. Thun is not on your list but it does have a nice historic center.
So the 2 bases could be:
- nearby Interlaken*
- nearby Lucerne
These are the 2 most visited areas by tourists so you'll find plenty of choices. Obviously at Swiss prices.
About the itinerary: apart from the big days from/to Paris, remain flexible. If it's raining, no point going to Jungfraujoch to end up in the fog. Rather go to Bern. Whichever ticket you decide to get allows that kind of flexibility.
On Day 1:
- Paris - Geneva or Lausanne high-speed train
- lunch
- Geneva/Lausanne - Montreux: sit on the right!
- Montreux - Zweisimmen - Interlaken
On the last day:
- Lucerne - Zurich in the morning
- put everything in lockers and go explore the city
- Zurich - Paris high-speed train in the late afternoon
*I tend to avoid accommodations in Mürren or Wengen as you "lose" 1h each way each day to get out of the area. Better to stay in more centrally located places such as Wilderswil, Interlaken or Spiez.
