Its an issue, even with paint, perhaps just do the back wall from corner to corner?

yeah, I have a feeling that's thwe only real solution. I can't imagine ending it before the round, or a little after it. in the middle seems strange too.

I have seen this with paint. You just have your plumb line, and you paper to that vertically straight line. Not around the corner.

It won't hold up well at the edge, no matter how plumb the line. Know that in advance. No clue what paper you're considering, but also no clue why no corners with that stair rail/newel post style. Very out of sync......

I'd paint, add art : )