The PE902 is the Glacier Express. Premium ‘tourist’ train, all reserved seating, which runs all the way from Zermatt to St Moritz. It has big panoramic windows, and offers Excellence Class.

All the other trains, including the R43, are just regular timetsbled trains operating over the same track. No reservation required. They won't go direct to St Moritz - you need a couple of changes. These are definitely the trains you want!

It's offering you the 1023. That's a perfectly legal connection, and you'll make the connection if the train from Milan is on time. But the Eurocity can sometimes be a bit late so the 1123 is a more solid connection. No harm in trying for the 1023 though.

I'm 99% certain you'll take the 11:23. Just to inform you on the punctuality of this EC : it never arrived less than 15 min late in the past month. It is routinely delayed by 20-30 min for 2 reasons : works in Italy, border control in Domodossola.

Always worth trying to get the 10:23 but no need to run across the station... simply get off the train calmly, go to a supermarket for picnic and get the 11:23 train.

Sounds like the Glacier Express is super expensive. I assume the suggestions you gave earlier about choosing 1st class, not in the panoramic cabin, relates to the Bernina Express, not the Glacier Express

Yes, the Bernina Express is slightly different. The same train carries panoramic coaches (must be reserved) and regular coaches in first and second class.

The regular coaches are fine, with big windows anyway. And they don't need to be reserved.