Can't answer them all, but basically Karl had an affair, Susan ditched him and fell for this other guy, who had 3 other kids (of which Rachel is one) AND a terminal illness.
He died, Susan said she'd bring his kids up as her own, Susan and Karl got closer again. As for the other kids and Stuart, well they went the way of many neighbours people - moved away never to be heard of, maybe occaisionally mentioned, again.
I'll try to keep it short. Here goes (Deep breath)
1: since you last watched Karl had an affair with a woman called Izzy .Karl and susan Divorced . Susan Met and Married Alex Kinski. When she divorced Karl susan got the house.So Susan Alex and his Children Rachel and Zeke lived there .Alex got lukemia and died Susan got the kids and was later joined by Alexes older daughter Katya. Karl spit up with Izzy and got back with Susan .
2: Mal,Libby and Billy all settled down and moved away.
3: Stuart left to run his parents farm and was replaced by his brother Ned who was fed up looking after the farm.(Exhale) Ihope that helps.:-D