"Murren sits on a sunny terrace"
May be in high summer. Between mid November and mid January, there is no afternoon sun at all (whereas Wengen, has no sun in the morning). But that's common to many places in mountain valleys.
If you really want to have sun from early morning until late afternoon, you must go to Crans-Montana.

The Valais is furthermore outside the rain belt Kandersteg - Lucerne - St. Gallen/Appenzell - Oberstdorf - Garmisch - Salzburg, much more protected against rainfall thanks to high mountains on both sides (together with other inner alpine valleys like Vallee d'Aoste or Vinschgau) and therefore sunnier, drier and warmer anyway

- Montreux or Vevey
7/12 - travel to Bern>

You can take the famous Golden Pass scenic train on this route - at least from Montreux to Spiez, where you would change to the mainline train to nearby Bern - takes a bit longer than the quicker mainline route via Lausanne but is infinitely more scenic.

> "Murren sits on a sunny terrace"
May be in high summer. Between mid November and mid January, there is no afternoon sun at all

Yes, we're talking about summer. The poster travels July 4 - July 13.

In mid-June it should be almost sunny the whole day and much of the night! And when the stars do come out in such a secluded place oh my you never saw so many stars in your whole life! Incroyable!

As I said before, you can easily do a day trip from Montreux to Zermatt, no need to stay overnight - and in this case you could make a last minute decision, depending on the weather.>

With the new Lotschberg Base Tunnel it is also possible to do a day trip to Zermatt from the Interlaken area - though a problem with Murren is that it is remote, adding about an hour each way over say Grindelwald or Interlaken itself.

day tripping to Zermatt from either could be very expensive - getting to Visp and back and the Visp to Zermatt and back could cost a lot I suspect - is it worth it for a fleeting look at the Matterhorn IF it is out - weather can change quickly here.