You are overthinking this deck building. The material may cup regardless of the measures taken that you describe. Buy full size 2x6 and you will get less cupping.

Buy the proper screws for the material you purchase, not necessarily stainless. Ask.

Hello RGB42,
You have some very good ideas about your deck and some very real concerns. Here are a few thoughts you may want to consider before building your deck:
1.) Garapa is a very strong high density hardwood decking. Stainless steel screws are the correct fastener for hardwood decking.
2.) Screwing the face of your deck boards into the deck joists provides the strongest fastening method. There is no stronger fastening method for exterior decking
3.) While screws may help minimize some potential cupping, they will not eliminate the potential for cupping entirely. Proper moisture management is the best solution here.
4.) Proper moisture management for a deck means
a. Adequate ventilation- your idea for ventilation on three sides is the right answer.
b. Minimizing moisture below the deck- many reputable contractors will grade the area below the deck to drain properly away from the structure, put landscape fabric down on the ground and then cover the fabric with 2-3â of crushed stone for drainage.
c. At 16â above grade, you are already extremely close to the ground.
d. Failure to properly plan your moisture management will lead to regret- so I would suggest that your plan includes a preventative solution.
5.) Since your area has more moss than grass, mold, mildew or algae are very likely to occur at some point. Why wait? Again, an ounce of prevention is worth a pound of cure. There are several deck sealers that should help minimize this. The sooner you apply it, the better your deck plan will take shape as you envision it.
I hope this information is helpful and wish you the best of success with your decking project!
-Chris

Here is a link that might be useful: Deck Installation Tips Blog

Thanks for the feedback guys. Since cupping seems to happen mostly due to moisture coming up from below, I may apply a coat of NeverWet to the bottom of the decking boards before fastening them down. Looks like it just hit the market, but it's a bit pricey. Anyone use it yet? You can check it out here: http://www.homedepot.com/p/Rust-Oleum-Stops-Rust-18-oz-NeverWet-Multi-Purpose-Spray-Kit-274232/204216476

I understand there is moisture below a deck and good ventilation will minimize moisture loading, but I can't see how moisture below a deck may effect the wood when the upper portion see's regular rain fall and sometimes snowloads.

It doesn't take much cross ventilation in a crawl space to protect joists that have zero treatment non cedar do not rot away after 60 years.

No need to seal below the decking.

We have a pressure treated dock that has sat in the water for 25 years here.

And 16" off the ground with 3 sides open is plenty of ventilation.