Hardiebacker not entirely on top of tub edge
I just installed 1/2 Hardiebacker along the stud walls of my tub surround, after putting up a vapor barrier. On two walls, the CBU rests basically on top of the tub lip that sticks up from the edges of the tub. But on the wall with the plumbing, the CBU is just thin enough to fit between the studs and the tub- it doesn't rest on the lip. It's really close, and between the thinset and the subway tile we are going to install, water would roll off of the tile and into the tub. But...
1. Do I need to lay another 1/4 Hardiebacker over this 1/2 Hardiebacker in order to have the CBU rest on the tub lip? Is there another way to shim the 1/2 out a bit to have it rest on the lip?
2. (a little unrelated)- can I use ordinary drywall mud and alkaline mesh tape to seal the seams, or should I only use thinset to do this? If the latter, can this wait until we mix thinset to begin tiling?
Thanks so much in advance!!!