I spent another hour crouched over my engine yesterday, with very satisfactory results. With minimal effort, the shaft slid out of the coupling, and I pulled it out of the stern tube. Whew! The water-proof grease applied to the shaft during reassembly two years ago really paid off.
I slipped the first spacer between the halves and easily pushed the shaft out of the coupling 3/8 of an inch, as I pulled down all four bolts in sequence. After loosening the bolts and backing off the coupling with the additional space provided by 3/8 inch, I slipped a longer spacer between the halves. I only needed to tightened one of the bolts this time, as the shaft easily pushed out of the coupling. Still too tight to free without a bolt and nut to draw them together, I used one more (longer) spacer to push the shaft towards the end of the coupling. At this point, I was able separate the shaft and coupling by pulling back on the prop from the outside.
Additionally, I was able to reposition myself more comfortably over the engine, so wasn’t crippled when I straightened up.
Photographic evidence of successful removal follows below.