I booked the extra passes on Wednesday. The only glitch I had was a ticket issue of my own making: our APs expire right before we get there, and DSS can't get FPs if you don't have a valid ticket. And I'm here to tell you that promising them you'll renew doesn't work.

Also, Disney, for whatever reason, will not let you renew your APs outside the 60 day window. Which baffles me, because really?
You won't take my money now, Disney????
Anyway, once I got that issue squared away, it was indeed seamless. I had a spreadsheet of exactly what I wanted, when I wanted it, and the wonderful Megan at DSS got them all. The entire conversation was maybe 20 minutes long, and that included getting a reservation for a Pirate Adventure Cruise, so I'd say the FP portion was maybe 12 - 15 minutes?
In terms of two FPs for the same ride, I didn't even try it. I had no interest in jeopardizing any of my reservations, because I can't help believing if it's a glitch Disney doesn't want, then eventually they'll fix it. That said, Megan was WONDERFUL about guiding me through the process. i.e., not making 4 FPs in one park, including two Tier 1s, and 2 FPs in a different park; pointing out where I could get MORE Tier 1s, etc. She was the bomb diggity.
For the fireworks, you do have to tell them which ones you'll be viewing, and your name is then placed on a reservation list.
It's funny; when they first announced this, I didn't think we'd have any interest in it at all. Turns out it's going to be a WONDERFUL addition to our trip!!!