They do not guarantee a night dive for 1 diver every night - no one does. They guarantee night dives on specific nights of the week, but not every night.Can't speak for martin but aldoroa says they have dive garentee and have no minimums if onr person shows up they will only take that one person
Also, with any reputable shop, if you have made advance reservations and have paid a deposit they will take you out on the morning dive even if you're the only one or they will make arrangements for you at no additional cost to you. MANY dive shops will do this, it's not exclusive to Aldora.
I think there is something lost in translation with the nitrox and the 80's. I think that means if you want Nitrox, you will have to dive with 80's, they don't fill the 100's with nitrox. ANd you don't need nitrox if you are not certified for it (actually they can't give it to you if you aren't certified) and if you're getting short bottom times, it's not necessary. It will not give you more air to breathe.
Have you been cleared by your Dr. for diving?
I recommend staying with a less expensive op for your first trip. Aldora and DWM are also not the only shops in town - but they are each at the far extremes of the scale - Aldora being the most expensive on the island and DWM being the most economical on the island - there are a lot of other shops and variables that set them apart from each other.
Work on your diving skills rather than relying on bigger tanks - the more you dive, the more comfortable and better on your air management you will be. Good divemasters can help you improve your air consumption over the week. My guys regular take people from 45 minute dives in the first couple of days to 60 minute dives by the end of the week.
And all I can do is shake my head and say "step away from the YouTube" re hurricanes. What were the dates on those? Cozumel has not had a hurricane in over 12 years. Our reefs are not dead. The "extra" money it would cost you to dive Fiji would be at least a few grand per person more than coming here from TX. You're going to be blown away by Cozumel diving.
Have fun!