Getting the train tickets has been a concern of mine
As posted above, use either SNCF or trainline. Rail Europe often has higher markups and they also do not show all the trains.
We are going on the Best of Eastern France tour which starts September 24, landing at CDG on September 25
Hopefully this is a typo, but you are arriving a day after the tour starts? It's best to arrive a day or preferably two earlier. Stay in Paris, get over a bit of the jetlag, and then take the train which you're sure to catch because you're already in Paris. It is more challenging to go from CDG to Reims directly because you don't know if your flight will be late, how long the immigration check will take, how long it takes to get to the Paris station for the train to Reims.
We did this tour several years ago and really enjoyed it! One tip I'd add is that if you have a small lightweight pair of binoculars they're worth bringing. Makes it easier to see the detail on the Cathedral windows in Reims and also the views in Chamonix.