Just returned from my trip to Southern California.
Overall, the FP/Rider switch combo generally worked well. On GotG, the CM might offer you to do the rider switch at the elevator loading area (so the party can ride it back-to-back and do the child swap between the rides). But if you insist that you prefer to wait outside of the ride, they could load the rider switch on the tickets.
Regarding the question whether the waiting adult needed a FP as well, they didn't really care. But on the most popular rides running out of FP early, we had FP on all four tickets and asked for Rider Switch on the second ride (to get a third ride for my daughter and a second ride for one of the adult).
The CM were not always very clear regarding the return time of the Rider Switch.
The one advice I would give is even if you are not using the Maxpass (we didn't and we walked a lot to get FP), I recommend downloading the app and linking your park tickets to your account. This way, once you get a Rider Switch, you can see the Rider Switch return time on the app. And you can use the app to enter the FP queue (and sometime re-scan the FP when they merge with the stand-by). This way, you can have the waiting adult getting more FP using the park tickets.