2 separate 4-day trip in 1.5mth, which pass to buy

Welcome to the forum Cynthia! You'll need to buy separate passes for both trips, as there's no rail pass that's valid longer than one month. Since you planned quite some longer trips during both stays, the 4 day Swiss Saver Pass would definitely be the most convenient solution. For the itinerary you listed, you'll only need additional discounted tickets for your visit to the Titlis. All further trips are free, even the one to the Rigi, so you won't have to buy any other tickets. Simply board the trains, buses and boats with your pass and that's all.

For your second trip in March, the 3 day Swiss Saver Flexi Pass (which can be used on 3 consecutive days) is an alternative, as you probably won't travel that much during your last day in Geneva. It's a bit cheaper than a 4 day Swiss Saver Pass, but whether it's more economical in all depends on your travel plans for March 15.

Supersaver tickets can be obtained through the SBB. They're available in limited numbers and for specific trains only, so you're bound to specific departure times.

Please find all details, validity maps and purchase options at the Swiss Saver Pass and Swiss Saver Flexi Pass pages.

In case you're still looking for hotels close to the railway stations: these overviews of Bern, Interlaken, Zermatt, Geneva and Geneva Airport may be of help.