You have great points...I just prefer the redundancy and the feel of SM over BM.
I like the feel of sidemount over backmount doubles, too, but that is a different thing. We're talking about a single tank vs two tanks.There's no way I would dive twin tanks off those tiny cozumel boats in OW water, swimming against current at times, on easy 1 hour reef dives, in either sidemount or backmount. It's just totally unnecessary. And sidemount might be more of a pain in this case because I guess you will have to put on both tanks in the water in a current while the other divers are trying to get down to the reef before you drift away from it. Maybe you are much faster and better about getting configured than I am, (it wouldn't be hard!) but for me it would be a real pain. With the single tank you just back roll off the boat like everyone else and you're ready to head down. When you come up you can either climb up the ladder with your tank on (that's what I do) or slip off the BC and hand it to the DM. With a SM set up you will have to unclip and un bungee each tank separately, stow the long hose, disconnect the LP inflator, all while hanging on to the boat ladder or line so you don't drift away in deep water.
Then there's the issue of swimming along the reef structure, usually with the current but sometimes against it, with the added mass and drag of two tanks. Sometimes there are swim-throughs that might be tight for two tanks. On cozumel reefs there are lots of nice little nooks and crannies to explore, it's nice to have the maneuverability of a single tank. I even bring my little full foot fins if I can fit them in my bag. After so much cave diving in full tech gear, I love the feeling of just having a single tank on a minimal BC to dive in. I use a small backplate and a 17lb tiny wing.
If you're worried about redundancy, just sling a pony. I don't worry about redundancy on those dives, I am a pretty careful and competent diver, I never lose track of my remaining air, we dive in groups there, and I always have access to the surface. These are very easy, forgiving conditions to dive in. If you're worried about air consumption, keep in mind that you will likely be diving with a mixed bag of divers, many of which are not very good on air, and all of whom will be diving single tanks. Roberta's DMs are good about letting people come up independently in buddy pairs, so that helps. And you can get bigger tanks, they might be AL100s (which I hate), but maybe she could find you some HP100s which are very manageable. Any of these tanks is much less mass to drag around than a set of doubles in either SM or BM. And you will probably use significantly less air in a single tank on most of these dives. I would, and I'm a very good swimmer with low air consumption.
I love sidemount in the Mexican caves, because it makes entry/exit much easier (no carrying doubles up and down those long stairs and slippery rocks) and the configuration fits the topography of the caves, and of course you need complete redundancy to dive in an overhead. But that's a totally different dive scenario.
Sorry for the long post. This is just a suggestion based on many years' experience diving exactly where you are going.
