I am not a pro but have done my own tub/shower similar to what you have. I only hung the hardibacker down to my tub lip, not over it, then used 100% silicone between the top of the tub lip and the bottom of the hardibacker. The tile then goes over the tub lip to 1/8" above the tub, then silicone the gap. I shimmed out the side walls with 1/8" pegboard cut into 1 1/2" strips to better match the drywall. I also did not use a plastic vapor barrier under the hardibacker, but I did use Redguard on top of the hardibacker. Hardibacker only up to the shower head is fine. You can still go ahead and tile up to the ceiling.