I was under the impression that evenafter JMS had made the decision that the entire arc was too much for only Sinclair (and would have to bring in the character who became Sheridan) that he still didn't intend for Sinclair to leave quite so early?
He didn't intend for Sinclair to leave
at all. The original plan was to have the "Sheridan-like" character (who wasn't quite Sheridan as we know him because he didn't need to be) added to the story in essence to fight the Shadow War, which he would do with the advice and help of Sinclair. "Sheridan" would take center stage in that part of the story (basically seasons 2 and 3) with Sinclair coming more to the fore again in S4 and S5. (The Earth and Minbari Civil Wars and the founding of the alliance.)
JMS has often compared Sheridan and his part in the story to Aragorn in
The Lord of the Rings, the military hero who complements Frodo's Quest Hero. Imagine if at the end of
LotR Frodo had destroyed the Ring, raised an army in the south, ridden to the rescue of Gondor, married Arwen and been revealed as the long-hidden heir of Isildur and rightful King of the Reunited Kingdom.
But when he first conceived of the "Sheridan" character, it was as an addition to, not a replacement for, Sinclair.
As for "Anna" being either Carolyn Sykes or Catherine Sakai - it doesn't work that way. The independent star pilot character was supposed to get into trouble with Earth's megacorps, probably a way into the Shadow Tech and black projects storyline that JMS had already been planning. The "lost wife" story was
never planned for Sinclair. JMS's whole problem with Sinclair and the Shadow side of the story was that Sinclair
didn't have a personal connection to it and JMS had never planned one for him. So he couldn't very well have written the Sykes character and later Sakai with that in mind. Instead when he started to consider splitting the arc between Sinclair and another character he came up with the dead wife as a way to tie that character into the Shadow War in the same way that Sinclair had been tied into Minbar
since the pilot. At one point he considered make
Garibaldi into the character who later became Sheridan, and JMS may have put Lise Hampton in place to give him an option there. (Since the network was also pushing him for a hot-shot pilot type for season two, he briefly considered making this character the one with the dead wife, but decided it was a bad idea.)
Everybody who tries to sell the "Anna/Sykes/Sakai" angle conveniently forgets that Sinclair's women were very much alike and both merchant pilots, whereas Anna was nothing like either and merely a scientist on an expedition.
Regards,
Joe