If I load my existing navigo decouverte for a week when arriving at CDG, can I use it to reach Paris? or is the 10E extra? I am mostly walking around Marais, etc., and may not use the full 22E for a week, but if it includes the trip to Paris, it's cheaper than the carnet?
Yes, Navigo Découverte will cover train rides from CDG to Paris, as long as you're buying within the validity period of the current week for Navigo (i.e. buying between Monday - Thursday midnight, you can use Navigo the same week, else it's good only for the following week).
If you're buying & using Navigo within the same week's validity, it will always be cheaper than a carnet of Metro tickets (14.50€) + two tickets from/to CDG (20€).