Paris de Gaulle landing/train timing

Rule of thumb for catching a train at CDG is 3 hours after ETA. You would be pretty lucky to get through immigration and bag collection and make the 11:24. I wouldn't book it unless you were able to be relaxed about buying a walk up ticket or had a full price ticket that allowed last minute re-schedule. I don't know if that is possible with a TGV train. If your plane is early and immigration doesn't take 2 hours and your baggage appears timely, you have a shot, but odds are pretty good of missing the train.

That was exactly the intelligence I was looking for, thank you very much. Sounds like I should play it safe.

And then the plane will be an hour early and immigration take 20 minutes -- and you will be drinking coffee. But I'd rather do that then watch the train pull out of the station without me. For years most of my planes were half an hour to an hour late -- one was 5 hours ate -- but more recently the airlines are building in a little cushion so they are more often early to on time.

Is catching a 11:24am train too tight with a 10:05am land, with bag retrieval, etc?

Well, you can probably get better odds of pulling it off than the Jets or Dolphins being in Super Bowl LIV, but that's not saying much. Give yourself the extra margin, as you have indicated. Much better than spending 8-10 hrs on the plane wondering/worrying if you will 'get there in time'

On the same topic is it easy for a newbie to find/ticket that train from CDG-Paris and then Paris-Reims?

Although I have never bought a rail ticket at CDG, I have done so often in train stations all over the country. The vending machines have an English option and are easy to use if you are facing a significant line at the staffed counter(s).