If that tub is also to be used as a shower, you should tear out the drywall and install cbu...drywall is intended for DRY locations, and neither tile nor grout will keep it dry...a disaster waiting to happen (even if it is greenboard, which BTW, is no longer in the national codes, although some locales have not adopted it yet).
The tile should not be installed directly on the tub deck...there should be a gap there you caulk (unless you use something like Dilex from www.schluter.com). you should caulk the corners also, but Schluter has a tile-in expansion joint for corners, too if you want to avoid caulk everywhere.
As to the warp, is the tub lip ledger board straight, level, and installed at the proper height? Did you ensure the floor was perfectly level or support the tub in a mortar bed?