I have several dissectums growing in pots and one, which I excavated recently, is slightly younger than yours but thicker because it was grown in the ground. You tree is a lower graft than often seen on nursery dissectums nowadays and because of its age the bark hides it. Your tree is stressed from your degraded, water retentive soil, it was most likely already in decline before you made the air layer. Not to sound too blunt, however I have experienced this many times in the past and I'm positive that if you were to leave it like this until next year, it would be dead. You need to bare root the tree, give it a light root pruning (i.e the dead roots and a few of the largest lignified girdling and encircling roots) and place it into fresh soil (based on what you have- 2 parts bark /1 part pumice, no potting soil ,and make sure that you sift each component to keep particles in between 1/8 - 1/4 inch, this is even more important considering the size pot you have it in). Repotting and root pruning this late in the season will not kill your tree, however you need to cut the top and your graft off (which you would do anyway). This will preserve the bottom.
