Your plan is definitely feasible. After all, Switzerland is not very big.

For ticket options, see the thread: Travel Passes for Visitors.

Here's one suggestion regarding your route. You could make it an even more fantastic and memorable trip by not coming back the same way. Go all the way over the Gd St Bernard pass to Aosta, then head up the Aosta valley and travel via several cable cars from there over the Mont Blanc massif and down into Charmonix, France! From Chamonix, there is great train ride to Martigny. It's a route that my wife and I would like to try to do some time this summer.
This site has info on some on the cable cars, and if you click on the "Aiguille du Midi" link half-way down the right-hand side of that page, you'll see a graphic containing the route over the massif.

This would probably take an extra day, be quite a lot more expensive, and figuring out all of the transport in 3 different countries would be a bit complex, but that is all part of the fun! Anyway, it's just a suggestion because it was something that I was thinking about recently. Unfortunately, the cable car over the top doesn't open until the Spring.

Be warned that there is not too much to see in Martigny or Aigle, they are mainly just useful transport hubs for accessing the mountains. If you're interested in wine, then the chateau in Aigle has a wine museum with lots of artifacts and a little info about the local wine region, plus you get to wander around the pretty castle while seeing the exhibits. I believe Martigny has some Roman stuff, but I haven't explored it much.

Do you realize that you can also stay the night in the hospice (basic mountain hotel) that is at the top of the Gd St Bernard pass? Here is the website of the hospice, and their price list. The hospice has played a very important in the history of the Great_St._Bernard_Pass.