May I suggest you don't try and cram in too much. Your plan so far looks a bit non-stop.
I know Reichenbach/Kandertal very well (I was there only a few weeks ago) - see my pictures of the railway around Reichenbach taken last month
26/12 - Zermatt, Gornergrat RISK OF Poor weather on a one day visit
27/12 - Glacier Express from Brig to Chur and return on regular trains (my wife badly wants to do this)
28/12 - Lauterbrunen, Murren, Schilthorn Recommended
29/12 - Grindewald, Jungfraujoch Recommend drop Jungfraujoch (rather over-developed nowadays), and possibly Grindelwald as well (View of Jungfrau. Eiger etc excellent from Murren on a clear day
30/12 - still not sure what to do City (eg Bern) or castles around Lake Thun
So, here are my questions:
I feel like our itinerary is focused only on mountains, any suggestions to change something? And what to do on the 30/12? see my comment
How full will the attractions be at this time of year? Should I expect waiting queues? Any lifts directly associated with ski-ing may be busy, but usually only at the 'rush hour' times of day (morning, lunchtime uphill; lunchtime, afternoon downhill)
Since we will be with our own car, if we take the Swiss Travel Pass, do you think I should leave my car parked at our village and go everywhere by train? I was thinking of maybe driving to each city and parking, and from there doing everything without the car using the travel pass.
Leave you car at your accomodation. Reichenbach is served by trains every hour (even more frequent at some times of day), and the journey to key junction stations such as Spiez, Interlaken or Brig is 1 hour or less