I don't have any suggestions to offer on the best Swiss Pass to use, as that will require a fair amount of number crunching, and will be based on the specific sights you want to see while there. Were you planning to just buy P-P tickets for the Italy portion of your trip?
As mentioned in the previous reply, this is the route to Mürren....
- Train from Luzern to Interlaken Ost (1H:50M direct)
- Transfer to small Berner Oberland Regionale train for the short 20 minute trip to Lauterbrunnen (be sure to check the digital display on the side of the car to make sure it's going to Lauterbrunnen, as some of the trains "split" at Zweilütschinen).
- Upon arrival in Lauterbrunnen, walk across the street to the Cable Car station, walk up the stairs and board the next Car to Grütschalp. Trip will be about 15 minutes or so.
- Upon arrival at Grütschalp, exit the Cable Car and board the small BLM mountain railway, which will be waiting.
- Sit back and enjoy the scenery! The train will likely make a short stop at Winteregg and then continue, and you'll arrive in about 20 minutes or so.
- Upon arrival in Mürren, walk up the stairs to street level and carry on to your hotel.
The entire trip may or may not be covered by a Rail pass. You might sent a note to the Rail department at RS tours for more information on the various passes.
You can easily visit some other villages, especially Gimmelwald which is a short downhill hike. If you want to visit Wengen, Grindelwald or other places on the other side of the valley, you'll have to return to Lauterbrunnen and travel by train.