Just a follow up for the tub. I wasn't quite convinced about the plastic sheeting because it would not be directly attached to the flooring itself, so I decided to go to the local Lowe's here and grab a few pieces of 1/4" Hardi-backer (aka concrete board). I installed the concrete board on the plywood subfloor, then the thinset in between the tub and Hardi-backer. Smartest way to go. One, if you have to tear the tub up you don't have to tear your subfloor up; two you can safely secure your tub and reduce any and all flexing and cracking without worrying about subfloor rot.