Hi Pat,
Great questions.
Let me take them one at a time.
1. How early should I get there? (10:50am train Paris-Bordeaux RT)
There is no cutoff time like for planes. If you are on board before the doors close, you're fine. The trick is to guess how long it takes to get to Gare Montparnasse itself, taking account for any of the possible troubles you may experience: getting lost on the Metro, delays on the Metro, etc. I'd aim to get to the station 30 minutes before departure.2. How do I find a yellow box to validate ticket?
There'll be plenty near the train platforms at Montparnasse. Simply walk from one train lane to the next and you'll run across one near the signs signifying which lane you're on.3. Are the train #'s clearly marked? Dont' want to get on the wrong train and end up in Strasbourg
Train numbers aren't marked on the trains themselves, rather, on the departures board which indicates a lane for your specific train, as well as on video screens on the lanes themselves. Gare Montparnasse has several large departure boards showing train departure times. It will show each departing train with a lane letter or number, i.e. where a given train is parked. To find your train on the board, first check your departure time against those listed on the board. This will usually narrow down the possible trains to one or two. Departure times are exact down to the minute and are not "rounded off". Next, take a look at the train number on your ticket and match it to the trains you've narrowed down on the departures board. This will tell you exactly which lane your train is parked at. IMPORTANT: The Departures board only shows trains that are departing soon, i.e. the next 20-30 minutes (and often with much less lead time). If your train is not marked, do not panic. Your train should show up on the board eventually as the time to departure nears.4. All the TGV trains are on the highest level in the station? (2?)
In France they count ground level as 0, so one up, level 1, is where your train will be. The main hall is quite long with another area for trains off to the end and left, so keep that in mind in case your lane number is 25 or beyond. 5. Unless I pay $200 USD each way, I cannot access the lounge car. Do people get up and walk around or stay seated the whole time?
Feel free to get up and walk around. There are separated areas on each car near the door where you can take phone calls and stretch your legs. There will be a space for luggage and a few "jump seats" for folks without a reserved seat.6. Are people friendly or are they not going to be too happy sitting next to a tourist?
I wouldn't worry. People in France are generally very courteous and respectful.7. Who/When/Where do they check your ticket?
There's no set schedule. At random (and sometimes never) a controller dressed in black/purple, often with a hat, will come around and ask to see tickets.8. What is the Gare St. Jean station in Bordeaux like?
Sorry, I've never been there, so I can't say.9. What happens if I cannot find a yellow box and don't validate my ticket? Am i going to get put off?
Most likely the controller will use his ticket puncher to punch your ticket and he'll probably shake a finger at you. He does have the right to fine you, but that's up to the controller.10. Is there any possibility of seeing sights out the window ? Or is there not much to see at 200MPH?
You'll see plenty. The train line is not very close to many things so things out the window will appear... not really that fast.Enjoy your trip.
Ben