You can add the mesh tape when you are tiling, or do it before. If you do it before, you're more likely to end up with a speed bump. It's not like you're trying to make a pretty seam like on drywall...you want the mesh there as reinforcement and as long as it's embedded in the thinset under the tile, it doesn't matter when you put it on with one exception - if you add a paint on waterproofing, you need to do it first, let it cure, then add the waterproofing.
The amount of thinset underneath the tile as it goes from the Hardie to the drywall at the outer edges of the tiled area usually isn't an issue...backbutter those tiles then only run the notched thinset to the edge of the hardie...as long as the majority of the tile is bonded to the Hardie, the little on the drywall won't be an issue if you've put a bit on the back...you want it supported without a gap, but it does not need to be the full depth as the majority of the tile on the Hardie. Otherwise, if you put the full depth of thinset on the drywall as well, unless you push fairly hard, your outer tile will not be in line with the rest of the tile on the wall.
The bigger issue may be your wall/tub lip and what's behind the Hardie. Unless you are going to use a full waterproofing layer on top of the Hardie, you do want a vapor barrier behind the wall, lapped over the tub's tiling flange to direct any moisture that might get there into the tub, and not down the wall under it. There won't be much, but there it's possible the Hardie may become damp if used frequently...you want to keep that off of the studs.