Chad, you mention this is also a shower; ideally you would use a cement board, such as HardieBacker, in the tub surround, but if you're set on using drywall, be sure you use the green MR board (Moisture Resistant).
The flange overall can be handled several ways; many don't do anything, letting the rock float from stud face to the flange. You can shim the stud faces, though, to bring them out to flush with the flange; to do this, use drywall shims available at lumberyards & building centers.
As to the corner, you may need to shave away some rock from the backside of the sheet on the first side of the corner to accomodate the curve; the next sheet abutting the corner should fit against the first without needing to be trimmed. Keep your material 1/8" to 1/4" up from the ledge of the tub.