Please note that although I have been to Switzerland nine times on vacation and use the trains on my vacation, I do not have as good a knowledge as a local person.

First, if you haven't seen it yet, you may want to look at: https://www.seat61.com/trains-and-ro...er-express.htm which has answers to some of your questions.

For the Glacier Express itself, you need both the seat reservation and the ticket. For the regional trains that follow the same route but aren't the "glacier express", you don't need a seat reservation but you still need a ticket. The seat reservation is CHF 49 and it's not refundable. You buy the seat reservation from https://www.glacierexpress.ch/ but you do not have to buy the ticket from them. Note that https://www.glacierexpress.ch/ sells only the more expensive kinds of tickets: there are less expensive ones it doesn't mention or sell. You can buy the ticket at the station on the day you take the train if you want to or you can buy in advance and save some money.

The difference in the classes on the Glacier Express is, I believe, related to the seating, not the windows. The windows in all classes are large, picture windows. The problem is, as Nico40 says, that the windows on the Glacier Express do not open. This is an issue if you want to take good photos. Some camera auto-focus systems do not work well through a window and the window won't be 100% clean.

You do not mention the possibility of bad weather: you may find that you will travel on a day when it is raining and you will not see any scenery out of the windows. For the glacier express, you have to buy the seat reservation well in advance if you want to be sure you travel on a particular train and can sit together but you don't have to buy the ticket weeks in advance: you can buy the ticket on the day of travel (or, for example, a day or two before travel but after you know what the weather forecast is).

That you have a person in your group who is mobility impaired is an issue that requires careful thought and I am not the right person to provide information. Although I personally would always take the regional trains instead of the glacier express along the same route, you may not want to try that because you will need to take five trains and changing trains four times may be far too much with a wheelchair. I don't know how the glacier express deals with travellers in a wheelchair but presumably it is going to be a better experience on the glacier express with no changing trains than changing trains four times because you are using ordinary regional trains. According to https://www.glacierexpress.ch/en/faq "As a wheelchair user, how can I make a reservation? Please contact our Railservice: by phone on+41 (0)81 288 65 65or by sending an e-mail to[email protected].."

For food on the glaicer express see: https://www.glacierexpress.ch/en/faq. If you don't like the options or are on a regional train, the simple solution is to buy food from a grocery store and take it with you. Many Swiss railway stations have grocery stores very close to or inside the station.

Personally, I would always try to travel on the entire route from Zermatt to St. Moritz. Near Zermatt, there is a section of the line (near Randa) where you get a very good view of the Weisshorn and associated glaciers. At the other end, a highlight for me is the Landwasser Viaduct near Filisur.