While Sudderth's node is one guess as to what happened to Jeffrey Sinclair, he is unfortunately wrong.
Michael O'Hare, the actor who played Sinclair, started having worse symptoms from a debilitating mental illness during the filming of the first season of Babylon 5. Towards the end he had to take producer and writer J. Michael Straczynski aside and explain what was happening and why he was having mood shifts, delusional behavior, and had issues remembering lines. The main issue was fighting with the crew due to paranoid delusions.
With O'Hare's permission, Straczynski talked with other key actors and they all worked together to get O'Hare through the end of the season. Even then, Straczynski did not abandon O'Hare. He brought him back to finish out his new character arc, even though it was very difficult and Straczynski didn't know which version of O'Hare would arrive on set. They worked everything out and got everything on film during the second season.
This is why the actor who played Captain Jeffrey Sinclair left the series and why Bruce Boxleitner was brought on to play Captain John Sheridan.
Michael O'Hare died in 2012.
Iron Noder 2017