Navigo Pass Decouverte

Hello,
I was on the Ratp website looking up information, I have an old Navigo D pass that is still valid, and it says I can load up to two (2) day passes onto my card, or a weekly pass....is this correct? Or am I misunderstanding the information. I am trying to decide whether or not to load a weekly pass onto my card since I will be there from Wed to Sunday. (If I stay in Versailles or Paris)If I decide to just load a daily pass, can I load the airport RER ticket on there as well? Or do I purchase that separately? Still in the planning stages actually...at first, I was going to go straight to Versailles from CDG,stay in Versailles, and travel to Paris by RER a few days, but now I am also debating whether or not to go directly to Rennes from CDG and stay there for a few days before going to Paris (since I definitely am going to Mont St. Michel, it seems a better more efficient and less expensive way to do it). I originally was doing a day trip to Mont St Michel (yes, I know it is a long day but that does not bother me, and I can easily take a train from Montparnasse early am and get to The Mont by 9 am or 10am, leaving plenty of daylight to explore and enjoy.) But the more I think about it, it might be better to go to Rennes directly from the airport and stay a few nights so not rushing. Any thoughts or advice? I already booked a hotel in Versailles and one in Paris to lock in my dates but have free cancellation so no worries there.
Cheers