"It lodged in the bone just above the spinal column."
It was actually lodged in a bone OF the spinal column near the spinal cord. The vertebrae make up the spine and the cord runs through them.
It would appear that the lodged WW would have worried that bone back and forth by rubbing of the scapula as the buck walked because there is very little space between the top of the shoulder and the spine. The arrow probably snapped off from this action of the shoulder blade.
Dr John's theory that inflammation of the spinal cord caused partial paralysis sounds just right to me. Arrow heads that hit the spine seem to really stick in the vertebrae, often so hard that they cannot be pulled. Torquing that vertebrae as he walked had to cause trauma.
You did just right to take a break and check in with the Boss for inspiration. It is never wrong to ask for guidance, asking to find a specific thing may be pushing it, but you must have had some chips to cash like JC said.
I bet you subconsciously waited for the deer to step forward with the close side leg. This opens the heart and lower lung for broadside but covers the upper lung / spine area from high angle close shot.
Good shooting, good follow up, good story, great WI buck. Way to go Whip!