@Phaedrux I restarted from zero and followed the calibration instructions on the Duet3D wiki to no avail. Everything is square, tower endstops are at the same height, no slack in belts, magnetic rods are all the same length and ball joints are clean.
What it appears I am getting is a skew towards the Y tower. On a square print, the other three corners appear square, however the right hand side of the print curves inward from the top right and bottom right corners. The curve is sharper towards the corner closest to the Y tower.
I've done the G32 calibration and am not seeing huge variances between the before and after and the bed mesh seems reasonable. I can only assume that it is a hardware issue of some sort rather than software. A print oriented between the origin and the three towers is within approximately one tenth of a mm on all axis (Z +0.22, Y +0.1, X +0.12 measured with a mitutoyo vernier caliper). I used the 'Advanced Delta Calibration' tool from Thingiverse (https://www.thingiverse.com/thing:745523) as my primary test piece. Haven't run it through the tower angles yet though, so maybe that will shed some light.