Until you are Enriched Air Nitrox certified, take Nitrox out of your vocabulary. Since you are a novice diver using a 100 cuft tank will probably give you about 8-12 more minutes bottom time. At your stage of diving that can be significantly better. Based on everything you have stated, I think you will be much happier doing 2 or 3 days of diving with DWM, take a day off and then do 2 or 3 more days of diving. Since I am NOT a doctor, I can't knowledgeably address diving with high blood pressure. I've never had that problem, but I find that I am more relaxed underwater than when I'm dry. EVERYONE is different. As they say on here YMMV (Your Mileage May Vary).
You didn't state whether or not your fiancee / wife is a diver. The implication is that she is not. If she isn't, please remember that it is your honeymoon which is a couple's trip. Five or ten years from now you don't want to hear her say to someone, "I spent my honeymoon waiting for my husband to get back from having fun!". Having supported a couple of divorce attorneys for a while, I can speak to that subject.

Cozumel is a great dive location for all levels of diver. Because it is primarily drift diving, you can practice buoyancy and body control which are important to air consumption. The more relaxed you are, the less air you consume, AND the lower your blood pressure will be.
On the 100 vs. the 80. A 100 cubic foot tank filled to 3000psi has 25% more air in it than an 80 cubic foot tank filled to 3000psi at the same temperature (100 - 80 = 20. 20/80 = .25 80 * 1.25 = 100). Some guys named Robert Boyle and Jacques Charles passed a couple of laws which makes this true.

Dive safe and enjoy your hobby.
Cheers - M²
