What is the best time of year to do a major chop?
--Mid-Fall or early Spring.
Should the tree be reduced over the course of several seasons?
--No. I think that successively weakens a tree. Do it once, do it right, and it will recover.
Since this tree is a nursery tree it will likely require a lot of root work. When would be the best time to repot the tree, before or after the trunk chop?
--I would chop in the Spring, and wait to repot until Spring '16. An undisturbed root ball should encourage a strong response to drastic top pruning.
When it comes to the actual chop do you leave a stub or cut flush right away?
--I would leave a stub of about 1.5x the width of the chop to allow for dieback. By the fall, it should be safe to carve it to look like the attached photo...flush with the exception of a small point in the middle. This helps shed water and slow rotting while the callus rolls. Once the callus gets to the point, carve it away.
What kind of after care is required?
--None, the newly-exposed needles may sunburn a bit, and the water uptake will be slowed.