Rereading my post and piecing things together I think I have an answer to my own question.
Q: What is the strategic advantage if GM holds out on the Z28 and banking on the convertible to maintain a 7k unit gap?
A: GM realizes that both convertible and special models eat into existing coupe sales, staggering their release (Vert and Z28) will capitalize on a slight overall sales increases per model, gradually without cannibalizing the coupe. This also doesn't create two different kind of hold outs and won't drastically affect the normally slow Q1 sales. I'm still not sure about a fall release of the Z28, I am still thinking early summer would be best.