I can answer some of the questions (take it with a grain of salt though). Starting with Jack, he was mentioned in Jamie´s first full episode. You hint something, you have to feature it in the future, a rule for most supersoaps (with "Dynasty´s" exception). They were very tentative with Jack´s personality in the beginning: when he reunites with Jamie, he´s cearly a hustler. The problem is that Jamie had turned out to be almost a goody-goody, and her opinion about Jack might have been a product of her moral code, a bit à la early Pam.
I think that what Capice (and partly Katzman before leaving) had prepared for the future "Dream Season" was a bit different from what we got. I highly doubt that Jack was going to be a substitute for Bobby, rather than another cowboy in the Ewing family, and a pawn in the Angelica storyline. Probably his affair with Jenna was going to be that, an affair, and not the one-sided attraction to Jenna that we saw. We might add the aids-scare caused by Rock Hudson´s participation on "Dynasty".
I´m not sure if Charlene was fired before Patrick decided to leave, but I don´t think so. What I´d say was decided no matter what was the final reunion of Bobby and Pam, and Katherine´s return to stir up trouble, following the pattern of Kristin in her last year. There was a rumor that is somewhat confirmed by Lucy´s Hot Biscuit saga: Lucy starts the season hysterically complaining about her horrible family where "nobody cares about anybody." She often repeats that she isn´t interested in rich people´s lifestyle anymore ("Ha!", Miss Piggy comments).
What the rumor said was Season 8 would continue the waitress Lucy story with her meeting a cult who shared with her that newly found hatred towards the rich. They turned her crazy and she let them entry the property during an event (a wedding?). Then would follow what you might guess: a massacre that decimates the family. Funnily enough, the week prior to "Swan Song" that other massacre had happened on "Dynasty", which makes the whole theory rather unlikely. Another source (capsule synopses) hinted a "tragedy in the aftermath of Lucy´s wedding, with a fast car heading towards the family Ewing (this sounds a lot like Sydney´s "apparent" death in the Season 5 finale of "Melrose Place").
Patrick had asked for certain things regarding his departure, so I don´t think it was a last minute plot for the finale. All cast had signed a 7-year contract in the beginning, hence Charlene was easily axed when they couldn´t get a decent story for her. Victoria stated that she noticed the show in Season 6 that she would leave two years later. I´m sure that the news of Patrick leaving and Bobby dying were decisive for her to renew her contract for two more "final" years, since the latter would give Victoria more chances to shine as an actress.
That year was a turmoil for the producers also for the Donna Reed fiasco, BBG knocking on the door, and Linda Gray being fired for asking to direct one episode. If it hadn´t been for Larry, maybe our sweet Sue Ellen would have ended at the "Rock Bottom Alley". It was a hell of a season for the "Dallas" honchos, all in all, with the show being surpassed (allegedly) by ·Dynasty", and then it became worse when they replaced "Ugly Baby" with Joshua Harris. No more "eggs and toast (beech)"...
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