Sounds like you covered your basis. Just curious, how did you put mastic at the screw holes? put mastic on the top of the Screwholes? On the slab?
It also sounds like the plywood was not raised at the edges, only at the centers giving you an opportunity to weight it down.
Good thinking in giving the plywood a few days then testing for moisture content. Hope all works out for you. Have fun.
Merry Christmas.

Thanks, Ray, for the help. I drilled a counter sink in every hole right after drilling the actual hole. This gave me the ability to "zero in" on the hole when applying the mastic (from a caulking tube). Otherwise, it's a bit difficult to insert that mastic aimed in a 5/32" hole. Also, I would try my best to not stand on the plywood as I squeezed in the mastic, so thre was no pressure build up, and the mastic would go down the hole and under the plywood.

Do not go with AC unless it is ACX it must have the X as the X signifies exterior rated glue.

the suggestion offered by KLS should not be taken lightly. The difference is huge. X is important.