As a Jack fan, I also never want him back. Like Titan said, he hasn't been written well in over 20 years. If he were to come back, agonizing history would repeat itself. If you want to "see" him again, write a fan-fiction and move on with your life.
As for Vivian, Louise Sorel said she would never return after that last albatross of a stint. Stop banging that drum. No one is listening.
I don't think these two returns will be anyone that hasn't returned occasionally in the past. I.E Shane, Kim, Melissa, Austin, Carrie, etc. Nothing to get so worked up over.
I've never heard of this Tiffany, so this write up/history lesson was well worth. I have, however, heard of Carla. She seems like a real piece of work. There's a video montage on YouTube showing a lot of major deaths and as soon I instantly identified the clip in my head when you described it. It was interesting to see that Grace and Sam crossed paths with her. I do wonder how the show chose them, of all people, to bring back when they did.