I've done just one quick dry fit with the lestello and I just didnt have enough material in the corners to pull off a miter. Even with a cope I might have to play with the spacing within the lestello. We'll see once the floor is in and I finish the wall end with the rondac, what space I have to work with.
The last remaining problem I foresee is what I'm going to do at the bottom end pieces of the shower tile where it will meet the bull-nose baseboard tile. Especially on that wall end next to the toilet where it will meet the vertical bull-nose of the wall end. I try not to think about it too much, it just gives me a headache. I'm hoping for a revelation when the time comes.