I would think it WOULDN'T be the activity pool. I know they don't use that one for the Tulum and Akumal weddings... so I would assume it'd be the same for Coba. But quite honestly, I've never seen a poolside reception at the Coba, so I can't really help.
My wedding was scheduled for 1pm and was moved to 3pm the week before we arrived... but really, 2 would be fine.
DJ, I used the resort DJ and I loved him. Worked out just perfect and I didn't have to deal with outside vendors.
Photographer, well, that's all based on your personal style and budget, so I wish you best of luck! But, as I did with my wedding, I would HIGHLY recommend flying someone in. It was so awesome to have three days of coverage for our event. The pictures are priceless and it ends up to be very cost effective.
Beautiful weather... don't worry, it could be the worst storm in the entire world, but when you marry the man of your dreams, it'll feel like the sun has never shone so brightly.