How can I make the corner tile transition less conspicuous with an out-of-plumb wall?
When you build stairs that land on an out-of-level surface, you plan from the lowest point. The bottom step tapers as the slope rises. The same concept applies here. Ideally you'd have laid out your grid from the widest point of the wall. 

I'm not sure if you're asking about the task of cutting tiles or the end result, but ideally you'd have set the tile for the top row in the same horizontal position that you've set the bottom row. This would've resulted in tiles growing larger as you worked upward, but you'd never encounter the end of a tile. Thus, you avoid skinny tiles and have a less obvious taper.

Carry on as you are. Skinny tiles are still tiles, and even though they're difficult to cut and set on plane, your project will look fine. From inside the shower with the curtain closed and shampoo bottles everywhere, not many people will notice the taper. 

If it's really going to bother you, consider cutting in a cove or bead accent strip, or simply a vertical run (soldier course) of the same tile at each side of the corner. A diamond wheel in an angle grinder would work to create the channel.