Is the tub also used as a shower? If so, you're lucky the drywall isn't in bad shape. If it isn't used as a shower, then yes, your plan should work. Leave the cbu a little bit off the top of the tub and caulk it. When installing over the drywall, you'll need longer screws than normal to catch the studs...no real benefit to screwing to the drywall inbetween - you'll find it hard to figure out when to stop since the cbu is so much denser. The only time you use thinset under cbu (or at least the only time it is needed) is if it is a floor, and then, it is necessary for a reliable install. You'll get some good info on tiling at www.johnbridge.com.