Definitely go directly from CDG if the timing works out. You will be jet lagged so limiting your transfers, etc, will make your life easier. If you have to kill a few hours at CDG, you can go to the lounge at the Sheraton in Terminal 2 and get something to eat and drink in the meantime.
I have taken the train directly from CDG to elsewhere in France several times and I usually give myself less time than suggested between flight arrival and train departure because if I can limit my time waiting around in the airport, I accept the risk that I could miss my train. It has always worked out for me thus far (obviously, this may not always be the case) and I rationalize the risk because usually, I have bought my ticket early and have gotten a really good price that I am willing to lose (this is when I am travelling without my family - I will eat the cost of 1 ticket but would be less excited about the cost of 4). I am also at the stage in my life where I can afford to buy another ticket, at the last minute, if necessary. When I was younger, I likely would have thought twice about the same scenario.
Enjoy Reims (lovely place!) and the rest of your tour next year!