Versailles train stations Navigo card purchase?

I’m sorry that I don’t know the answer to your question, but the Navigo Easy with individual tickets might be a better deal for you than a day/weekend pass depending on how many trips you plan to take. We arrive on a Sunday morning so I’ve been making the same calculation, and I believe the day pass (or weekend for only one day) only made sense if we took 4 trips in a day. You’d have to get the more expensive day pass to include Versailles. See https://parisbytrain.com/paris-metro-pass/ for a good explanation.

ND cards can be purchased at any station in the Ile de France and any metro station in Paris.

Paper tickets will be cheaper than a Navigo day pass for the round trip to Versailles, if Versailles is the only thing you do that day. The fare includes the metro connection in Paris but not bus connections. You should buy tickets for the round trip in Paris: it does not matter if the ticket says "Paris-Versailles Château RG" or the other way around, so just get 2 Paris-Versailles Chateau Rive Gauche tickets per adult.
The youth ticket is a great deal - EDITED see below, not so great anymore

The round trip Paris/Versailles is 8.20. it is 14 Euro for the day pass that includes zone 4. so you would have to take a lot of metro trips in Paris to make the day pass cost effective for an adult. I'd just get paper RER tickets and if you need additional metro rides in Paris, they are 2.10 each on the machine for a paper ticket. The Ticket Jeune for 5 zones is about 10 Euro and so worth it if you take even one extra metro ride in Paris.

14 euros now?!? Wow, hadn't checked the price since the 1 Jan fare hike. That Navigo day pass will almost never be worth it then.
And the youth deal got a lot worse, too.

Thank you for the detailed information! We'll use paper tickets for Sunday.
Super excited--we fly in six hours!