StoneLifter,
Your musings about the Z feature...
This plan of Pete's is excellent - http://www.avebury-web.co.uk/stone_numbs.html
I understand that the Z stones are thought to be a later addition, but what strikes me is the alignment in the opposite direction - straight through the dead centre of the whole henge, AND through the oldest feature, the cove.
What this means I don't know but a line that has three significant reference points - and perhaps FOUR if you're right, is beginning to look a bit deliberate, to say the least. I think you're dead right to be suspicious.