RE: Great Planes Ultra Sport
If the horizontal stab is warped you are going to want to straighten it out. Like you said, wet it down. But instead of pins lay down some wax paper (or plastic), wet the stabilizer, put it on the board, another sheet of wax paper, and then use a large book on it with more weight on top of that. Then give it plenty of time to dry out.
As for the cowl. I almost never use the ABS cowls that come with the kits. Never had any luck with them. I order fiberglass aftermarket cowls. They hold up a lot longer in the long run, IMHO.
You've got one of the best flying planes out there. You're going to love it.
Hope this helps
Ken