No one rail pass will cover everything you might do. I think you're a good candidate for an Austria-Switzerland pass, priced at $270 for 4 days of train travel within two months, but it does not cover the Munich-Salzburg leg, and gives you 25% discounts (not full coverage) from Interlaken to Wengen and on most trains, lifts, and boats in that area.
You don't need seat reservations for the Rail Jet or any other train in your plan, if you don't want them. A rail pass can add to that hop-on flexibility, since you could change your mind about the train route or take a later train, still covered as far as Interlaken (with 2-country pass) or as far as Wengen (with Swiss Travel Pass).
When is the trip?
If booking the Rail Jet train leg in advance doesn't sound like a hardship, then you could also look for advance-discount tickets on the Salzburg-Zurich portion at https://ticketing.oebb.at/inet/pub/en/ticket. (For example, I see a September fare at €69 one way, or a late October fare at €39, down from the standard fare of €102. Or, you can pay just to the border crossing at Buchs SG station.)
If you buy that ticket separately to Buchs, then for the rest of Switzerland, a Swiss Travel Pass for 4 consecutive days costs $277 and covers you fully to Wengen, or on the other side of the valley to Muerren, and covers lower-level trains, buses, lake boats, and museums, but still just 50% off Muerren-Schilthorn tickets or 25% off Wengen-Jungfraujoch. There are other options, too, but I assume maximum coverage is meeting your idea of "easiest." This is obviously not a cheaper up-front cost than the Austria-Switzerland pass that I mentioned earlier.
Among all the places I've changed trains, Swiss stations have been among the easiest, with good on-time arrivals, good signage, informative staff, and in some lucky cases, the next train just waiting on the next track. You can also see some station plans/maps at http://www.sbb.ch/en/station-services/am-bahnhof/railway-stations/trafimage-maps-station-plans.html.
