Not a scream but an update that I hope will prove useful.
First, the tooltip alerting me to the fact that Hindenburg can't be promoted doesn't indicate whom he would be bypassing (i.e. who I could appoint in his place) - only that he's ineligible for a GHQ appointment.
Second, I can bypass him and appoint a GHQ West without receiving the NM penalty. But I do still have to wear a massive VP hit -around 1900 points.
Third, I can't appoint a further GHQ. All existing three star generals have the promotion option greyed out and the "cannot be appointed" tooltip.