I think they should be there as well, however, I remember just after Sandy walked that I read somewhere that he had said categorically that he did not want to be there for the final show, even though he was contracted to be there.
As for last year's housemates, I think this is very unlikely as the producers wouldn't want to detract from this year's show by reminding us all of last year. This is exactly the same reason they use for banning housemates talking about the previous years shows.
