As you say, it depends who you speak to, but the only people on the production team who had a say in how it ended and whose comments really count were the producer and the script editor/episode writer - and the latter says he delibberately wrote it so that those actors who signed for another season could be brought back, and those who didn't could be presumed dead without too much fuss, after Jan Chappell's changing availability had caused him headaches at the start of season four.
I think he's got to be treated as the 'authoratitive' (quite literally in this case) source.
What happened with Jan Chappell? I've never read anything about her leaving - just assumed she wanted to move on.
Of course there's lots on Gareth Thomas's final appearance as Blake
