To complete this one, our decision was made for us by SNCF, which trimmed service on the RER E to do summer repairs, so the section between St-Lazare/Haussmann and Magenta was closed. We ended up walking out of the Hilton a bit late at 10:10, hopped onto a local bus for 3 stops to Auber (free because of Navigo passes so why not), and got on the 10:20 Roissybus 2 minutes before departure. As everyone predicted, there was zero traffic at that hour on an August Sunday, and we went directly out of the city, up the Rue du Faubourg-Saint-Denis/Rue de la Chappelle to the A1. Kind of a good experience to see the outer parts of Paris I had never seen before, actually.
Got to T1 at 10:54, and to our destination of T2E at 11:05, exactly 55 minutes after walking out the door of the hotel. Those who said it would take an hour were quite accurate. It was definitely a good choice because we didn't walk more than a few hundred feet with bags and didn't have to negotiate any long staircases through the Metro/RER system. Thanks all.