What do you guys think.
It is hard to see in your photograph, but I think I see deflected nozzles (small curved vertical lines of ink in the missing color just to the right of the left and right sides of the empty pattern). You may need to look at the pattern under magnification to see this.
Based on my experience you have a failed head, not a clog. You have 2 key indicators: 1) a continuous block of missing nozzles that gets larger with more attempts to clear it, 2) deflected nozzles with "delayed" firing.
I had pretty much the same thing happen on my 5 1/2 year old 7900 in the Green channel. Up until that point, I had never
had a missing nozzle which could not be easily cleared. I installed a new Ink Selector (all five dampers) which did not fix it. A 72 hour soak with PeizoFlush fluid in the Green channel did not clear it. I installed a new head (did all the recommended adjustments) and now have a perfectly functioning system again. Note: If you do replace your head, and if you build your own profiles, I highly recommend re-profiling all your papers. Replacing a head can actually change color characteristics in the same way that changing media settings might.