Which train pass to buy? Versailles Q’s?

It was a decade ago, but we used the Navigo weekly pass. It is worth it even for a little less than a week, and it includes metro, buses, and trains to CDG and Versailles. https://parisbytrain.com/paris-metro-pass/

Definitely NOT Versailles on arrival. You will be in a haze.
Navigo weekly unfortunately only works for Mon-Sun weeks, but it'll still be cheaper to buy single tickets for CDG-Paris on Sunday then a Navigo for Mon-Thu, than to buy the whole-region, tourist-oriented Paris Visite Pass which costs over 60 euros for 5 days, 65 perhaps.

It is enough to purchase a RER return ticket for about 7 euros, and there is no line to skip : most people queue for the castle while you very much can get straight away to the garden.

It isn’t common to see people picnicking at historic or cultural sites in France. We had lunch at a very nice cafe on the grounds. If it were me, I would definitely not head to Versailles the day you fly into Paris. Your flight may arrive at 8:30am but by the time you get through Immigration and Customs you’ll be lucky to be on your way to the city by 10:00am and that trip takes at least an hour depending on the location of your hotel...and the Marais is right in the center, so yes, an good hour.
Just a side note, we didn’t go to Versailles until our 10th trip to France...and while we loved the gardens, the palace was crowded and honestly, kind of wretched excess as my husband put it. To me, it was over-rated compared to other things to do in Paris. Have you thought of training to Giverny to visit Monet’s house and gardens? It is a little longer train trip, but beautiful in May...there will be so many flowers in bloom. Plus, the Museum of American Impressionism nearby is a delight. The train goes to Vernon which is a nice little town with some good hotel restaurants. You can walk or taxi to Giverny. There is a bus, but you have to make sure to verify the return trip!

Versailles has two areas where picnics are allowed, both away from the main gardens. I’ve seen local families and couples do this, but it is not common.

http://en.chateauversailles.fr/plan-your-visit/facilities/picnic

There are a variety of food options in the grounds, much closer to the formal gardens than the picnic areas are.

The Navigo Decouverte will be cost effective and very convenient for Mon-Thurs including the trip back to CDG and the trip to Versailles -- and all other travel you do Monday on. You will need a postage stamp size head shot; 25/30mm. Just crop and print on your computer at home. You can get the card (5 Euro), assemble it and charge it up with a weekly pass at the metro station near your hotel and be all set for Monday.

You cannot picnic in the gardens at Versailles but you can picnic on the wider grounds and at the base of the gardens by the canal is a good spot and there are kiosks selling very good ice cream and drinks down there. Lots of people picnic. The gardens are an extra fee on. days the fountains run and IMHO worth it, but you need to plan well as they only run a short time morning and afternoon.

It's really going to depend upon how often you will be using public transit to get around Paris, which is generally very walkable. If not all that much just buy your RER tickets to Versailles and to CDG at the station.

How wonderful for you all to lend advice, thank you.

To clarify I will be flying into Paris from a 3 day stay in London so I’m hoping jet lag will have passed but I can’t predict but something I should be prepared for. And I’m taking in the advice about possibly passing up Versailles now and instead focusing just on Monet gardens which was a goal anyway. Taking it all in.

Giverny us lovely but will of course not be on the Navigo Decouverte if you get that as it is in Normandy not the Ile de France. If you don't go to Versailles then a carnet of paper tickets you share with your companion around Paris plus the tickets on the RER to CDG is probably your best transport bet.

Since you will be coming from London, Versailles is feasible, though perhaps logistically awkward, on arrival day. I think the train would be quicker than flying to Paris, but perhaps you have already bought tickets.

It is definitely a big outing. The grounds are huge (and worth exploring) There are many places for picnics, and places that sell sandwiches and refreshemnts, on the grounds.

I'm not going to say its better or worse for a day trip than Giverny. That's a judgment call. Expect the gardens there to be really crowded but lovely.