The Glacier Express and Bernina Express run quite far from Interlaken yes. Instead of the crowded and expensive trains which can sell out weeks in advance, it’s way better to use the reservation-free hourly regional trains running on the same line. They’re less busy and have pull-down windows too :)
I’d take part of the Glacier Express route on your way to Como :
- Interlaken Ost/West - Spiez
- Spiez - Brig
- Brig Bahnhofplatz (front of the station) - Andermatt Glacier Express route (on a regional train)
- Andermatt - Göschenen
- Göschenen - Locarno very comfy and modern train (on the scenic Old Gotthard Railway)
- spend a few hours in Locarno
- Locarno - Como direct RE (avoid the EC to Como which has a mandatory seat reservation)
Use SBB app for all connections
For your other questions :
- you do not need to (and you shouldn’t) activate a pass day in advance
- as soon as you board one train, it’ll count as one pass day whether you take 1 or 10 trains
For your other days in Interlaken there are much nearer very scenic train rides :
- Interlaken - Lucerne
- Interlaken - Spiez - Zweisimmen - Montreux (Golden Pass route)
- take boats on Lake Thun and Lake Brienz (pass valid)
As no trains in Switzerland require seat reservations (except for the Glacier and Bernina Express which can be avoided, see above), you can board any train you’d like at any time of the day. You’ll maybe in the mood for Lucerne… or Montreux and you can decide last-minute, 2 min before boarding the train !!! The great flexibility of Eurail pass