Yeah, sometimes you just can't get around it. DWR is a whole lot of challenges, some people find it easier just to applique down the arcs onto whole cloth instead of trying to piece it in. One thing I've found is that templates/patterns for just one size unit rarely are truly correct -- typically your end pieces will be trimmed a bit. That's why a lot of the pieces have one center for multiple parts and then a L and R for the enders. Sometimes it isn't us who is off!
With arcs though, it can be a surprising difference on which piece you have on top. I have to mark my seams and center points/hatch marks carefully to get the desired results. I used to pinch press but now I use pencil on the back side and will cut a template so I can draw that seam to follow it if I need to.
The other thing that gets confusing is the disappearing nature of the seam allowances and where you actually want stuff to line up versus how it looks like it should end up....