Peter Jones (PeterBJ7) was spot on when he commented about your lack of communication from the dive shops as being the result of a lack of "office" support staff. For better or worse, I'm not sure if you can call many businesses on Ambergris and "always" get an answer. As suggested, I also agree that it would be a great idea to book only your morning dives for the first day. Actually, if you're arriving on Ambergris before 3pm on the day before you dive, I wouldn't pre-book at all until you size up the situation.
Amigo's is certainly the largest shop on the Island with the biggest boats. It may be the source of your AL100 tanks but is also likely to have the larger numbers of divers on their boats. If I had talked to the rude woman that you had talked to, I don't think I'd have them even close to being on "my short list" Personally, I'd use them for the longer atoll trips but I'd stick to smaller shops for the local diving. Hence my previous suggestion of Island Divers.
As far as making a mistake, don't make that decision until you see what develops. I don't think you'll be sorry. Of the many diving destinations I've visited, with the exception of the Cayman Sister Islands, Ambergris is the only one that I've made multiple repeat visits to. With the right dive shop that meets your personal tastes, you're going to have some great reef diving. Combine it with some fantastic food and spend some time enjoying all the things that make San Pedro a great little destination. Visit Estelles for breakfast and Wild Mango's for dinner. Have some Ceviche' at the Hurricane bar or the Key Lime Pie at Blue Water Grill. The Sushi is pretty good there too on Tuesday's and Thursdays. Belikin tastes good everywhere. Though it will be much different than your Kokanee, Moosehead or Molson.
If you're staying at Mata Rocks, I'd certainly rent a golf cart for your visit. Walking back and forth can certainly be done, but you are bit south of town and I think it would get a bit old. Having a cart will increase the spontaneity of your travels. Besides, its the island way of life.
Relax. Enjoy the trip and maybe I'll see you there. I'll be the one staying at Banyan Bay, diving an AL80 with Island divers, eating breakfast at Estelle's and drinking a Belikin while driving my golf cart. Ken :42: