I’m not sure how you could mount a rear wheel backwards. However, I still think that the wheel should be able to be corrected. The Ultegra hub has about a 3mm spacer on the left side, so when I first mounted my wheelset and noticed the wheel was offset to the right, I assumed that the spacer needed to be on the right side. So, I removed the lock nuts and rotated the cones to thread the axle to the right. Then mounted the spacer on the right. I ended up with the wheel now off center to the left.

Hell, I don’t know, either I greatly over estimated the greatness of this bike mechanic, or something bizarre is going on here.