Arriving at ORLY midday on a Sunday. We'll need to get from Orly to Paris (Orlyval or Orlybus). I think we should purchase a Navigo Easy and put Orly Bus fare on it as well as maybe a "carnet" of 10 metro rides. We also want to go to Versailles one day, so we'd need a Zone 1 - 5 Day Pass? (but can't put that on at the same time as the Orly Bus, right?) Finally, can we even purchase a Navigo on a Sunday? Thank you in advance for answering.
You can put Orlybus ticket on Navigo Easy card and also a carnet of Paris Metro tickets for each Navigo Easy card (you need one card per traveler as you can't share a Navigo Easy card).
Versailles is in zone 4 so you can buy a Navigo Day pass 1-4 zone and put it onto Navigo Easy card or a Navigo Decouverte card.
You can buy Navigo Decouverte cards on any day, but if you're putting a Navigo week pass onto Navigo Decouverte from Friday-Sunday it won't be valid for travel until after midnight Sunday / Monday morning.