I had a very similar trip in September 2014 and it was great. I went right from the airport to Luzern for 3 nights, spent 4 nights in Mürren, then 3 nights in Lausanne, then went right from Lausanne to the Geneva airport to go home. So I like your plan very much!
I agree that you don't need to change hotels from Mürren to Wengen. Particularly with 5 nights total, you'll have plenty of time for activities on both sides of the valley.
For my trip I used the Swiss Pass. The reason I chose it over the Half Fare card was the included Museum Pass; although I didn't get to as many museums as I thought I would, I did still get a lot of use from it (particularly in Lausanne). The thing I didn't realize until I got there was how convenient the Swiss Pass is. With the Half Fare card, you have to buy tickets every time. With the Swiss Pass, you just hop on and flash your pass when asked for a ticket; you only have to buy tickets for things not fully covered (in my case, only the Schilthorn, the Allmendhubel, and the Jungfraujoch). Now, it's not difficult to buy tickets, and often you can do it from a machine, but it was still a great luxury not to have to even think about this most of the time.
So, I recommend adding up the cost of the transit and activities you definitely know you will do, and seeing how they compare with a Swiss Pass vs the Half Fare card. If it's close, get the Swiss Pass for the convenience.
For most people, the must see sight while in Lausanne is the Chateau de Chillon near Montreux. As someone who doesn't love castles and crowds, my experience there was not so happy; however, I liked Montreux. Closer to Lausanne, my best memories are of the Museum of Outsider Art (Musée de l'Art Brut) and seeing Audrey Hepburn's grave in nearby Tolochenaz. I also enjoyed the Olympic Museum.
Note that while a boat ride on Lake Geneva is going to be pretty, it won't be nearly as dramatic as the scenery on the boat ride from Luzern to Vitznau and back from Weggis to Luzern (which is why I didn't do any boat rides on Lake Geneva). While there are several boat and lift combinations available from Luzern, I chose the one to Mt. Rigi over Mt. Pilatus, merely for financial reasons (the entire trip to Mt. Rigi was covered by the Swiss Pass, while parts of the Mt. Pilatus trip were only 50% off). Mt. Rigi was a great half-day (more like 2/3 of a day door to door). If you have a Half Fare card, you can take whichever you prefer; with only 2 days I wouldn't do both.