To get the design to match up at the corners you pretty much have to ease what you have in. Yes, it would fit better if you took off one of the half triangles. If you've made the other side units and have similar issues, I'd consider extending the floating border as being simpler to alter. You could take it off, or maybe a narrow really dark additional inner border.
If you can make the corners so you can end at the valleys instead of at the halves of the triangles, you can just trim down that floating border slightly.
I know you are quite the detective in figuring out the problem, I think double check the seam allowance on the bottom, especially where the triangles meet. Sometimes when you are doing the seam allowance correctly it looks funky as you do it, so you adjust but it's not what you want. When we sew, small differences become huge when repeated because each mistake can be doubled by the two sides of the pieces.