(I don't see the Rome2Rio screenshots.)
Zermatt - St. Moritz - Milan is a long journey, too long to fully enjoy it. The Glacier Express is supposed to be a memorable experience but in retrospect you probably won't have good memories with such a long day.
Depending on the time of year you'll also miss the Bernina railway due to darkness. It's a real shame as it is even more scenic than the Glacier Express in my opinion.
I'd very very much suggest spending a night in the Engadin Valley : not in St. Moritz itself though, as it's expensive for nothing.
There are 2 routes from St. Moritz to Milan :
- fully by train via Tirano
- bus to Chiavenna + 2 trains
Eurail wouldn't cover the St. Moritz - Chiavenna bus. Trains are regional services, reservations not possible.
If you must go to Milan that evening : follow the Glacier Express route on regional services until Andermatt and then go southwards via Göschenen and Lugano. Much better experience and still pretty nice.
For your other questions :
Eurail is fully valid on Swiss trains (except ones to peaks such as Gornergrat). Reservations are not needed : log the train, hop on and sit wherever you want.
You need a 13€ seat reservation for Milan - Florence. Have a look at https://www.seat61.com/interrail-and-eurail-reservations.htm
Questions welcome. Please reconsider the Zermatt - Milan day.
@thibcabe thank you for your suggestion. If I were to spend the night in the Engadin Valley which town/ train station would I get off? I really want to do the Glacier Express train and have that experience. Do they allow you to get off the Glacier Express train at that Engadin Valley train station; or how do I get there?
Can I board the Bernina train in Engadin or do I need to go back to St. Moritz?
St. Moritz is in the Engadin valley so you could either go all the way to the terminus or stop in Samedan 10 min earlier. I don’t recommend staying there as it’s a ski resort for billionaires to show off (basically). It’s also mainly German-speaking while the nearby villages still speak Romansh, the fourth national language of the country! (Of course English is fine for a tourist.)
I’d recommend villages such as Pontresina or Zuoz. Look at pictures online and see where you can find accommodation. They both have frequent trains to and from St. Moritz.
Download SBB app for timetables and everything. There are hourly regional trains from St. Moritz to Tirano (Bernina line) stopping at Pontresina between others. Easy to reach the line from Zuoz too.
