How did giraffes develop their rete mirabile failsafe?
I think what leads to the confusion here is the assumption that the neck was evolved before the rete mirabile.  Giraffes evolved from creatures with shorter necks.  As evolution occurred nature was selecting for longer neck creatures (being able to eat more leaves) but at the same time creatures with longer necks had problems with blood pressure. 
At this point this would also not be lethal because we are talking about animals with much shorter necks, just in some way disadvantageous to the animal.
So over time necks got bigger, blood pressure went up, and blood vessels developed that could better and better deal with the change of pressure. Rete mirabile also occur in other animals, so they are not a rare evolutionary phenomenon of giraffes.