Hey, Sheriff!
Papa Hog's (in spite of the name) is just great. So is Alejandra's (la jefa) at this URL:
http://www.cozumel-diving.net/alex-alex/
Most dive ops on Coz use AL 80's, but you can ask them for special dispensation! I have only used Al 80's and steel 80's on Coz.
As for EAN, you again, need to ask them. I have never used anything but air whilst on Coz because I usually only dive two dives a day and rarely go very much below 100 ft on the first dive. So, EAN is not that valuable to me on the island. But folks I dive with do use Nitrox, so it can be done. One note of caution: sometimes it seems like it is not available. Maybe it's the spirit of "manana" (sorry, no tilde available on my keyboard).
I have not used the other two shops you mentioned, but Living Underwater has a fine reputation with folks I know.
The diving on Coz is, for the most part, relaxing and not Indiana Jones stuff. There are a few sites that are challenging (Maracaibo Reef) and there is the occasional strong current that can be like Dorothy out of Kansas. I am probably a wuss, but always carry a safety sausage and signal device on Coz just in case the current decides I need to go to the Yucatan! (That's a joke!)
The water will be considerably warmer than you are used to and there are really no wrecks (in my opinion) worth diving on Coz. However, it is some of the very best diving in North America and you will come home with a set of memories that will keep you warm all Winter. The fauna are spectacular--from the smallest trunkfish you have ever seen to some fairly spectacular nurse sharks and groupers. And the coral--well, it is like an underwater rainbow with all its variations. If you are lucky during the day you may spot a fish that is only found on Coz: the Splendid Toadfish. Its purple, black and yellow colors will knock your fins off! It is nocturnal, but can be seen during the day, peeking out from under the coral. Ask your DM to keep a lookout for one. If you do a night dive, you are more likely to see one.
Hope this helps,
Scorpionfish