I certainly believe a lot of things were foreshadowed by Joss and the staff years earlier. That little line just might have been foreshadowing season 7's events, but I always thought of it as foreshadowing the end of season 4, with the gang splitting up slowly, culminating in "The Yoko Factor".
I don't think so. Joss planned for the show to end at Season 5, he didn't plan for anything further until UPN picked up the show. I think it's just fans grasping at straws, the show wasn't that well thought out (look at Giles in season 6 and 7)
If you want an example of a well thought out show, watch Ronald Moore's Battlestar Galactica
Is Giles really a good example though? Even if Joss wanted to explore Giles more I'm sure he could have, but that wouldn't change the fact that Anthony wanted to live in London.