I'm not sure of the timing but I *think* I'd even read that O'Hare did his first conventions after the split had been agreed on and was a wonderful ambassador for the show.
He did ComicCon during season one, but also continued doing conventions up til the end of the show.
Marcus' death was meant to set up Ivanova to have a rebound romance with Byron (which is why there's a superficial resemblance between them) which would have caused a lot of dramatic tension when things with the teeps spun out of control and she'd have had to call Bester, wouldn't it?
That's the one scene I really wish Ivanova had been around for; after almost killing Bester earlier in the show in season three, then having her latent telepathy come to the surface by Byron in season five and in the end having to be the one to bring in Bester against telepaths would bring her story full circle.
And for those wondering how he could be revived, a line in the episode was cut indicating that he was put into cryogenic suspension at a moment between life and death.
Also, Joe had promised Jason that his character never would be killed after a practical joke in season 3; so he had to freeze him instead.
I always figured that if Andrea Thompson didn't leave the show there would be no sleeper-program twist, that the whole thing was invented for the sole purpose of getting Talia off the station and out of the show.
Joe said that the twist would have come about around the time of the Messages from Earth/Point of No Return episodes.