I just spent 40 minutes trying this on my 2017 -- programming a 3rd key. Still no mention in the 2017 owner's manual about the "2nd start button press on the last step."
Regardless, it didn't matter in my case. I don't think the 2nd press is necessary. Surely they would have corrected the manual by now.
Is it safe to assume that only two remotes will ever be active at once?
I'll go to the dealer, but I'm sure they won't be able to program a 3rd key. I think Ford engineers purposely made the sequence so convoluted that few humans could ever do it. The time frames are ridiculous. There's absolutely no reason why the software needs such timing. There's a sequence, do the sequence (without the silly timings), let the software keep track of the sequence, same result whether it takes a user 20 seconds or 1 minute to complete. And yes put a timeout for the whole process if necessary so the engineers can advertise "real time programming" (not even close) on their resumes.