I've been trying to follow this closely, in order to decide whether or not to purchase FTTF. The way I understand it is thus:
1. 16 day online check in is for platinum, diamond, and suites only (not FTTF)
2. Diamonds and platinum must stick to their arrival appointment time. Last I heard, suites do not. I have not read any reports on FTTF in this. However, the booking page for FTTF states that diamonds and platinum already have all benefits of FTTF. It would logically follow that FTTF must stick to their arrival appointment.
3. Wedding parties, suites, diamonds, platinum, and FTTF all board prior to zone A01 (I don't remember the exact order for these pre-A01 groups).
4. Without the 16 day check in window, or being able to arrive prior to the arrival time, the early boarding call for FTTF guests seems to me to be close to worthless. If you are there early enough to board with the FTTF call to board, then you got an early check in time, and thus would have gotten an early A zone anyway, meaning you would be on the ship only a few minutes later (15 minutes maybe?). If you get a later arrival time, you miss the FTTF boarding call. You do get the priority line for checking into the terminal, but, again, that saves you about 15 minutes these days? (I have no experience with these time estimates, it's just what I've read here)
IMO, if they are going to make FTTF stick to the arrival appointment, they need to give them the 16 day check in to make it worth it, but that would require making FTTF non-refundable at 21 days out or so.