Chris:
I took your data and made graphs for each region using the technique you used to display April volume for the central arctic and melting season losses.
https://docs.google.com/document/d/198DPkjMNK1YQ5ZzgSW41TuS3Xlv-O1eN5tZMMa4TdaQ/pubOn the one hand, the rate at which the trend lines have been converging for the arctic as a whole is higher than the rate at which the trend lines are converging for the central arctic basin. This agrees with your suggestion of a Slow Transition: the easy to melt stuff around the edges is gone, and we just have the hard stuff in the middle left to get rid of.
On the other hand, the graphs tell me a different story than what I'm hearing from you. In the central arctic, the summer ice losses were relatively constant as you point out from 1979 to 2002. Thin ice was completely melting away; thick ice was partially melting away. During the winter, what had been completely melted away reappeared as FYI. What was partially melted away became MYI, but wasn't as thick as before -- once we reach first year ice thickness, additional thickening is slow.
I would expect to continue to see this part of the trend. MYI still accounts for 40% of the ice. Ice more than two years old is still 24% of the ice. The ice still has a memory. The gap between the April high and the September low is precisely the thicker MYI that can still be lost.
In 2006 you suggest there is a step change. Although I'm leery of reading too much into random points on a graph, there are a couple of mechanisms that could produce a step change. The Kara, the Laptev, and the ESS started completely melting away by the end of summer around this time. The lack of protection on its flanks may have allowed the CAB to drift more easily when the wind was blowing to the south, or for waves to blow up on the CAB ice when the wind was blowing north.
http://onlinelibrary.wiley.com/store/10.1029/2011GL048970/asset/grl28491.pdf;jsessionid=0E87241F82E517C47B4B2F31B342C3F9.f03t01?v=1&t=hymji8qi&s=b6006ab31e2a3b651206af8a8a7d600901882caf&systemMessage=Wiley+Online+Library+will+be+disrupted+9th+Aug+from+10-2+BST+for+essential+maintenance.+Pay+Per+View+will+be+unavailable+from+10-6+BST.This paper suggests that it's gotten windier in the winter since 2002 in the CAB increasing the amount of ice drift in winter. Elsewhere, thinner ice seems more susceptible to drift, suggesting that when the CAB ice thins further, another step change could occur.
http://www.sciencedaily.com/releases/2013/01/130114092521.htm also suggests that thinner ice will be more easily exported through the Fram strait.
I would expect the momentum of the CO2 trapped heat to continue to provide linear increases in the volume of April ice lost during the melting season. On top of that, the various Arctic feedbacks look to be net positive: soot, more open water to trap sun and let large waves build up, thinner ice more easily transported, loss of protective ice in areas around the CAB. It's hard to see how, in the face of a harsher environment, sea ice volume loss would decrease for many years.
The ESS in 1990, the CAA in 1998 and 2010, the Beaufort in 1993... These all suggest that sea ice can collapse rapidly when the weather changes suddenly. Still, the CAA should provide an early warning. I wouldn't expect the CAB to be ice free until after the CAA started being consistently ice free.