Avignon TGV to Paris

When we arrive Gare D"Lyon can we go to the lounge for drinks as
Avignon has no lounge when we arrive?

Maybe, it depends on what kind of tickets you have. But a more important question in my mind is why would you want to do that?

Railway station lounges are usually pretty basic compared to airport lounges, since you don't need to arrive at the station hours before your train departs, they don't need to be that fancy. I've never been to the lounge at Gare de Lyon myself, but according to the Man in Seat 61 it offers comfortable seating, a water cooler and coffee machine, WiFi, phone chargers & toilets. If you want drinks, you can buy wine or beer on the train. Or if you really want drinks when you arrive at the station, visit le Train Bleu instead.

The lounges are only accessible to frequent travellers and to holders of the most expensive business first class tickets; I don't think you fall into these categories with your tickets. In any case the only drinks at the lounge at Gare de Lyon are coffee and packets of tea.

No, regardless. You cannot go to the lounge on arrival

Actually, reading this, I’m a little afraid you’re going to be disappointed all the way around

Right, regarding the seats: once you've chosen between a table and side-by-side or solo seats, they're all the same, basically. The end of the car close to the door is less desirable, but it's not a big deal, and there's more foot traffic upstairs in cars 2 and 3 (or 12 and 13) because they lead to the bar car.
All seats have tray tables because no seats face a "bulkhead" like you have on an airplane.

Don't expect anything fancy, it's basically just about having more room than in second class, and more recline.

And the food is awful in the train nowadays. You'll be much better off packing a picnic. Last Thursday I bought a juice, coffee, breakfast pastry combo. They used to serve fresh croissants in the morning. Now it's some cake wrapped in cellophane. You won't be getting a meal at your seat. You have to go upstairs to buy it.

The only difference between 1st and 2nd classes in the train is that 2nd class has four seats across 2+2, while first class has 3 across 2+1.

We are not disappointed because train travel beats flying on this route. I booked us lower level so no worries. I got the single seat and my Parents rhf Duo away from the door at the front first row.

I have been in the SNCF lounges at GDL, and in Lyon and other stations. Basically I go there for the bathrooms... I rarely have a lot of time to kill at a station. If you need to hang out non trivial times at a railway terminal you, or the railway, have done something wrong.

Anyway, when you arrive at GDL just head over to Chantilly as soon as possible. The trains on that route are local trains, and they run about twice per hour. So no need to spend longer than needed hanging around at the station. There are even direct RER D services from GDL to Chantilly...