Actually become proper housemates? And can win? Or do they just sit in a secret room with the evictee? What's the point of that?

No they're staying in the house for a week after staying in the room for a day and then leave on the 19th

I wouldn't have thought so.....no

Oh. But I still don't get it. What's the point of them going in if their storylines are going to be abruptly be cut short two weeks in?

It says on the BB website that: "And when the week’s up, the evictee and legendary housemates will re-enter the House before the REAL eviction on Friday 19 June." But that doesn't necessarily mean they'll stay. I doubt they will as they will presumably be being paid to be there. They all have agents now!