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

Thank you very much for your consideration 

So I can take glacier express from brig to Andermatt !,

if yes do I need to make a seat reservation please ? 

Brig And Brig Bahnhofplatz (where i can take the glacier express ) are 2 different stations ? 

What do you mean of front of the station please ? 

Your answer very very helpful thanks again 

Bahnhofplatz means train station square (in German) because the train to Andermatt basically leaves from the street. You need to leave the main building of Brig train station, there are signs don't worry. You'll see the small tracks very easily

Take a regional train between Brig Bahnhofplatz and Andermatt, they are reservation-free and run every hour on the Glacier Express route. Otherwise the special branded Glacier Express runs 2-3 times a day + you need to pay a 49 CHF seat reservation + it is often fully booked weeks in advance (or there are no window seats left)

In my opinion and I'm sure others will agree, it is better to take the regional train (it doesn't take longer btw). You can open windows and it's even better than the big windows for me :)