Switzerland itinerary for 13 nights

NC, The Golden Pass Route is Luzern - Interlaken - Spiez - Montreux. In doing the Golden Pass Route from Luzern, you go where you will be going later, then come all the way back on the same day - inefficient.

Since you need to go from Luzern to Interlaken to get to Mürren and the rest of the Jungfrau Area, I would suggest:

• Luzern 3 nights (as you suggest)
• Luzern - Mürren. This does the first half of the Golden Pass Route, then 4 nights in Mürren.
• Bern 2 nights, one of which is your day trip to Murten. But, Bern is only worth 1 day, and Murten only ½ day, I would consider taking another night from Bern and having 5 nights in Mürren.
• Bern to Lausanne, but don't go direct, take the Golden Pass Route, Bern - Spiez - Montreux - Lausanne.
• Lausanne 4 nights (as you suggest, this may be excessive).

I suggest you draw this route on the rail map of Switzerland: https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-sts-pass-gueltigkeit.pdf

I assume you are flying into Zürich. If you are arriving by train, depending on from where, you may be able to "cut the corner off" and eliminate Zürich.

Chris is right about Bern, Murren, and Lausanne.