I don't know the logistics of visiting the Montreux Jazz festival, but it looks like event tickets are still for sale online, not sold out: http://www.montreuxjazzfestival.com/en/program15.
If you start train travel on July 3 from Zurich airport to Lucerne and leave to Stresa, Italy on July 8, then that's 6 days in Switzerland. If you stay near Montreux and depart to Italy on July 10, then that's 8 days there and at least one of them spent on a train.
If you don't go to Montreux, then your main train rides Zurich-Luzern-Muerren-Domodossola add up to roughly $150. A pass for 8 consecutive days at $400 may be more than you need to spend and a flexi pass covering 3 or 4 travel days + Half-Fare Combi feature is not much cheaper.The value that you can get our of a Swiss Travel Pass or Half Fare Card will depend on how much sightseeing you fit into those stays in Lucerne and Muerren. Lower-level transport and museums can be fully covered by passes and higher mountain rides mostly discounted 50% (see the link), and these costs can add up.
If you choose the one-month Half-Fare Travel Card at $133, then you do at least save money over buying regular transport tickets along the way, even for the minimum amount of travel that is certain. Passes and Half-Fare Card are both sold at Zurich Airport train station (and you are almost running out of time for home delivery before you go).
