They make a special drill bit designed to bore out the old pipe from the fitting and leaves a nice clean hole to glue in new...I think it's called a rambit, but one of the pros would know for sure. No idea if you can rent one, or if you have to buy, how much they cost. If expensive, and you can't rent, it might be cheaper to get a pro to do it.
Unless this is a steam shower, you really don't need the cbu...drywall is plenty good enough when using Kerdi (and is the recommendation with that waterproofing membrane). If you haven't been to www.johnbridge.com for tiling, suggest you do.