Charles de Gaulle Airport – Le Chêne Gris – Disneyland

1. Yes, you're correct, Faremoutiers - Pommeuse is the closest train station to the Marvilla Parks, about a 1 minute walk.
2. From Paris CDG Airport you'd take the RER B train from either CDG T1 or T2 train stations to Gare du Nord station, transfer to Metro 5 direction Place d'Italie, transfer at Gare de l'Est to Transilien P train direction Coulommiers. (This is your route on Google Maps.) You'll need specifically Transilien P train in direction "Coulommiers" (that direction/terminus is important, there are 4 other terminus stations which Transilien P line train can end up and those are different than Coulommiers, which is the only branch which passes by Faremoutiers - Pommeuse).  There's no booked seating on this train, so no advanced booking is required nor possible.  The new Ticket Paris Region Aeroports which you need for the RER B train fare from CDG Airport to Paris theoretically covers your entire trip out to Faremoutiers - Pommeuse.  This ticket is brand new, like 6 days old, so we'll have to wait and hear whether it's working as intended and covers transfers at Gare de l'Est as we're imagining.  Transfers at large intercity train stations like Gare de l'Est were previously not possible on a single ticket, but apparently are now covered on the new Ticket Paris Region Aeroports. On the Transilien P train you can ride in any car/any seat. It's not reserved and is first come first served.
3. For Faremoutiers to Marne la Vallee (the Disney station) this is your best route, which requires Transilien P train then a bus.  Again, no booking ahead is possible, just ride both train and bus in any seat with proper fare.  You can buy fares for both trains & buses ahead of time from the CDG Airport train stations ticket offices / ticket windows when you arrive (they don't expire before usage/validation).  Just list out the exact routes you need on a piece of paper and provide that to the ticket agent and they'll be able to guide you & sell you which tickets you'll need and in what format (Navigo Easy or smartphone or paper ticket).

More information on the Ticket Paris Regions Aeroports will be coming out soon once I get the site updated in the coming days.