Hi Nick,
This is a complicated itinerary, so I understand the difficulty.
In my opinion, the following would be the easiest route:
RER B from CDG into Paris until Denfert Rochereau.
Change to Metro Line 6 (using the same ticket) direction Charles de Gaulle Etoile until La Motte Picquet Grenelle.
Change to Metro Line 8 direction Créteil and one more stop is Ecole Militaire.
An easy route to get to
Gare de Lyon the next day is this:
At Ecole Militaire, take Metro Line 8 direction Créteil/Créteil-Prefecture until Madeleine station.
Change to Metro Line 14 direction Bibliotheque François Mitterand. Get off at Gare de Lyon.
This is route is about 26 minutes. First train is at 5:37am with trains about every 5 to 7 minutes after that.
Enjoy your trip!
Ben