Well, with regards to whether it's worth losing a day of Coz diving, here's my story: I did my trip to Coz with four other people. Two of us were HIGHLY interested in diving the cenotes, and I had booked two days of cenote tours for my friend, my husband (who wasn't all that interested) and myself. We did the two days of tours and went back to reef diving . . . a couple days later, we all looked at each other and said, "Back to the cenotes!" And we blew off the rest of the reef diving to go dive the caverns.
It's a question of preference. I've done quite a bit of both and now I would only "give up" a day of drift diving when the port closes. It's good to do your home work and if the harbor master closes things up in Cozumel then pull your "cenote trump card". just sayn