They will be doing the "formal" portraits all during the dinner hours and if you do the PC dinning, I'm sure you can find some time to get some done. There are certain times though where you may have to wait in line longer than others, thats why Bob said to get down there around 5:00 to avoid the rush, but you should have plenty of time to get some taken.
Yes, with PC, you'd have different waiters every night, unless to request a certain table/section where a waiter you want to serve you works, just ask when being seated.
For the 5 "casual" nights, yes dockers and a golf shirt are just fine :-)
No, there are no documents for Cozumel and Grand Cayman, just for the Bahamas.
If you do order room service, yes, do tip the person that brings it up, its not your regular cabin attendant.
While there is plenty to coose from in the buffet for breakfast, the dinning room is a little different and you will be served with a waiter, etc. We always like to get at least 1 breakfast int he dinning room while cruiseing, but I think you will find the offerings in Horizon Court just fine.
Enjoy your cruise!