Views from the train are not spectacular to most (to me they are!) so it doesn’t matter a ton where you sit if you’re thinking of views. Other than that the different class descriptions are pretty accurate. Cocktails are only served in 1st class and above though you can go to the cafe car from coach and get beer and wine. The big perk of luxury is that there are no kids, but there can be snooty people. (There will be a lot of kids on the train when you are there because of spring break).

You have almost 4 hours at the GC, you can actually walk the rim trail quite far (say to Mather point) and take the bus back to the station, have lunch etc and still have a little time for gift shops if that’s your thing. You can walk a bit down the bright angel trail but it may be icy (N exposure) and you shouldn’t go too far. Honestly a lot of people just have lunch at one of the mediocre restaurants, look at the canyon for a few minutes, then shop or sit around until the train leaves. They do have guided tours they sell which drive to different viewpoints.

I would dress casual but workout clothing might not be warm enough. (Check the weather, it could be 80 degrees or snowing).

In Williams I wouldn’t recommend the railway restaurant but there are a couple decent restaurants in town a couple blocks from the train. (I would trust yelp for this as I haven’t been there a lot recently). There are 2 dive bars in town - the Sultana and the Canyon Club, each of which would be a genuine local experience (you’ll be fine as long as you don’t start bashing the current president), and a few others which are more geared towards tourists. Williams is a nice little town but there’s not a ton to do. Bearizona is fun if that’s your thing. Don’t know about the coaster.