TGV train from CDG airport

(1) No.
- Taxiing, getting off the plane, walking through corridors, immigration (the big unknown), picking up luggage, getting to the TGV station (CDG is a big airport), say 1-1½ hours.
- Intercontinental flight (I assume you are flying intercontinental), +/- 1 hour. Could be early, could be late.
So 2 hours is a gamble.
(2) No, tickets are for one specific train only. They have the train (date, time & train number), coach number and seat numbers on them. Like airline tickets, once the doors close they are worthless.
(3) Probably seats will be available, unless it is a particularly busy day, like a holiday weekend. But you will pay a lot more to buy on the day, and you may have to go 1st class (more cost). And it will take even more precious time to go to a ticket counter and buy a ticket.

CONCLUSION: If you like a gamble, book the first train. If you can get that train cheap enough, you may decide it wouldn't hurt too much if you miss the train, but you would still have to pay full price for the later train. Or by a cheap ticket for the later train. Your choice.

This may not answer your question directly, I have not taken a TGV from "the Paris Airport." Perhaps this will help.

First which airport? There are two major airports ... Orly and DeGaulle. I assume CDG?

Next ... here two very helpful websites for train travelers in France. Gare360 and Captain Train. Gare360 visually allows one to walk through almost all "le gares" (train stations) of France. You can find CDG's on the site. Next Captain Train is a train booking site that gives a wealth of information about your bookings, often more than the standard booking sites. They have a good help staff available online too for your questions. We have been able to book specific seats on it whereas other sites were limited. You might poise the question to the Captain Train people relative to your travel dates and needs.

Next ... go to this website for a wealth of information on traveling on trains in Europe. http://www.seat61.com

Traveling by train in Europe has been great fun for us. We wish you a wonderful trip.

You can get a ticket that is exchangeable called a TGV Leisure fare, but it cannot be exchanged once the train has left the station, which would be the case if you your plane were 2 hours late, or less and could not get through with a smart phone before departure. There is a 15 EUR fee and you would pay full fare for the next train.
The full fare ticket called a TGV Pro 2nde can be exchanged up to 2 hours after the train departs.
The train is unlikely to sell out. There are also trains 3 and 5 1/2 hours after that later train you were looking at.
What airline are you flying? United is in a whole different terminal and will take a while to get to the TGV station. If you are flying Air France, see if you can get a unified ticket with the TGV connection included in the fare that will cover any train to Avignon

Following on Sam's suggestion, the "airport code" for Avignon TGV station is XZN, which may only work in Air France's website.

Here is information on Air France coordinated flights and trains and how they work. Note that "your connecting time must not exceed 24 hours" which means that unless your arriving flight is more than a day late late, you'll be fine: http://www.airfrance.us/US/en/common/resainfovol/avion_train/reservation_avion_train_tgvair_airfrance.htm

Also look into flights into Marseille (MRS). You'll have to change, but MRS gets flights from lots of airports, not just the Paris ones. And this way, the airline handles your bags the whole way, and if you're buying on one ticket, your connection is again "protected" if your arriving flight is late.

If you do end up buying separate flight and train tickets, do take into account the arrival terminal. Delta, American, and Air France use T2, where the TGV station is. United uses T1, which requires a ride on the CDG VAL to reach T2. If you're using another airline, check here for their arrival terminal: http://easycdg.com/airport-guide/which-terminal-information-paris-cdg-airport-charlesdegaulle/

Definitely go for the 5 hour unless you are comfortable with buying a second ticket on arrival. Everything would have to go perfectly to make the 2 hour departure. It often takes 15 minutes or more just to get off the plane. Immigration can easily take an hour -- it might take 15 minutes or it might take 90 depending on how many jumbo jets unload before you.
Of the last 10 trips we have taken to Europe 8 have arrived late, one by 5 hours. 5 hours is rare, but an hour not, and half an hour common.

Luggage can be very slow arriving. And then you have to find your way to the station. You might make it but odds are at least even that you wouldn't and once the train has left, you cannot exchange a ticket even if you have a flexible ticket.

I would have probably arranged this trip by flying into the south of France on the original ticket, rather than trying to do a train so shortly after a Paris arrival. That way your transfer would be protected and you could take the risk of a short connection.

If you don't have your ticket yet, Sam's suggestion of an Air France combo ticket is excellent. They take care of everything, including transfering your baggage.

FYI, the other airport doesn't have a train station the way CDG does, just a local line, so we all knew what you were asking. No problem.

If you only have carry-ons, then there is a good chance you'll make the train. We recently made it from CDG to Gare Montparnasse within 2 hours of the arrival time. But there are so many variables, such as late flight arrivals and long passport control lines, that make it a gamble.

THANK YOU ALL for the helpful, and thoughtful, replies and suggestions. Well ... looks like the 5-hour layover is the big winner, so that's what we will do. In retrospect, of course, we should have flown into Marseille, but got an unbelievable deal on WOW Airlines to Paris, and didn't know at the time exactly what our itinerary would be.