Releasing a BE seat into Z doesn't hurt if economy oversells; either the Z seat is taken (via SWU/miles) by somebody in Y, freeing up one economy seat, or releasing it has no effect.

I think this issue is really the outside GAs not following stated Delta procedure. All people on airport standby for SWU upgrade clear first before non-revs or other upgrade methods (op-up). This was stated policy when the "new and improved" SWUs were rolled out back when SMI used to interact more frequently with the forum. The airport upgrade list procedure was supposed to avoid/prevent situations where people got stuck in the back holding an SWU when seats are available.