Gare de Lyon has a slightly confusing layout. It has two sections, called Hall 1 and Hall 2.
Hall 1 has platforms with letters, A to N. Hall 2 has numbered platforms 5 to 23, odd numbers only.
See plan here: http://www.garedelyon.fr/wp-content/uploads/2013/09/plan-gare-de-lyon.png
Only the French could think of this system.
Aim to arrive at the station about 30 minutes before your train starts. The video screens will list your train departing from "Hall 1" or "Hall 2". Go to the correct Hall, and wait. There is limited seating. Unless you are lucky you will have to stand.
About 15 minutes before departure, the video will show the actual platform. See video here: https://www.youtube.com/watch?v=iFVbOVx9McU
Walk to the platform. You will need your ticket to go through the ticket barrier to access the platform.
Find the correct coach and get on. You can walk from one coach to the next. But it is quicker with luggage to walk along the platform to the correct coach. Walking through the train you will be pushing past everyone else trying to get on.
Are we expected to drag all the luggage on ourselves up the stairs and leave the luggage in a rack on the second level then find our seats, or is there another area at the boarding level of our car where suitcases can be stored?
Yes, between when you get to the station and when you take your seat there is nobody to help you. Travel light.
Neither deck is at entry level. From the door there are steps down to the lower deck. See video here: https://www.youtube.com/watch?time_continue=1&v=KrXUqOPU9wc
Read this page about TGV's (includes pictures and videos): https://www.seat61.com/tgv.htm
There are limited places to buy food for the journey at the station. If possible buy your food earlier and bring it with you.
Once our tickets have been confirmed etc, are we confined to the one car or is there access to walk through to other cars?
You are thinking planes and airports again! There is no check-in, your tickets are never "confirmed". You get on, you sit down. You can at any time walk the length of the train. But trying to walk through the train when people are trying to get on or off will involve pushing past people.
Tickets are checked by the on-train ticket inspector while the train is travelling.
In Conclusion: This is actually simpler than you think. You are viewing it compared with the airport process, it is nothing like that.