Thanks for the link. I finally got around to reading it.
I remember that it was rather jarring at the start of the second season to find that Sinclair was gone. This finally explains it.
What a pity! Even though I grew to really like Sheridan, B5 would have been even better with Sinclair all the way through. I wonder what Sheridan's quotable lines and monologues would have been like coming from Sinclair.