I'm planning on re-tiling the shower in our bathroom. The shower stall sits
in a corner of the bathroom, and there is a cathedral ceiling above it, 12
feet or so from the floor. Looking at the shower door from the outside, the
wall on the right extends 8 feet past the end of the existing tile, and the
wall on the left extends only a few inches past the end of the tile.

Don't know if this picture will turn out, but here's what the bathroom looks
like from above.

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I suspect the greenboard behind the tile has deteriorated and needs to be
replaced. Cement backerboard seems to be the way to go. However I'm
uncertain how to transition from the backerboard to the existing drywall in
the rest of the bathroom.

Where should the backerboard be stopped on the right wall? In the middle of
the last column of tiles, so the transition is hidden underneath the tiles?
(Part of the last column of tiles is outside the shower door, so moisture
isn't so big a problem there.) One problem is that the stud won't line up
with the transition point. Do I slide in a stud at this spot, attaching it
to the adjacent studs? The cathedral ceiling goes up 12 feet at this spot,
and I don't particularly want to rip out all the drywall above the shower to
attach the stud at the ceiling.

The back wall is totally in the shower, so there's no problem there.

The left wall extends just a few inches past the current tile and is
finished off with a drywall corner bead. Seems I should take the
backerboard all the way to a new corner bead, and I need to tile all the way
to the corner instead of stopping a few inches short. If I don't, I've got
a couple of inches of backerboard that I have to cover up before painting.
Don't know how I would do that.

Finally, what to do up above? There's a 12 foot cathedral ceiling above the
shower. Do I stop the backerboard a few inches below the height of the tile
and cover up the transition with the top row of tile?

Thanks in advance for your suggestions.

you make the transitiong from backer board to sheetrock with joint compound.
They are the same thickness so it isnt a problem. Just make the last pass with
joint compound about a foot wide and sand smooth.