We’ll be taking the metro & RER from Invalides to Paris Disney. One option is to get off the metro at Opera and walk within the tunnels (correspondence) to the Auber station to get the RER A. I watched this walk on YouTube, and it looks like there is a turnstile at the Auber RER entrance. If I get a zone 5 ticket at Invalides, do I use the same ticket for the turnstile at Auber?
Yes, one ticket gets you to Disneyland from anywhere in Paris. You will also need to use the ticket to exit the station at Disneyland.
On the metro you only need to put the ticket through the stile once on entry; one ticket will take you anywhere it goes regardless of zone. ON the RER you use it entering and exiting as the trips are metered; if you haven't bought a ticket to the exactly RER station you are using outside Paris you can't exit and the fine is 35 or 50 Euro on the spot depending on whether you are viewed as having a wrong or no ticket. The system 'knows'. So in your case you enter the metro with the ticket to Marne le Vallee and then when you get to the RER station in Paris, you use the same ticket to enter the RER there and then again exiting at Marne le Vallee. If you have any issues, staff can help. ALWAYS keep the validated ticket until you are through the exit and out of the system and then throw it away. You can have the ticket inspected at any time and be fined if you can't produce it. An acquaintance of ours, who doesn't listen when women explain things to him, paid this fine last fall because he threw the ticket away once he was through the stile and into the metro. It cost him 50 Euros on the spot. We had explained the system to him before he left. We get checked about once a week; our record was 3 times in one week.