i thought they would be let out after their contract was up but now thinking about it they keep mentioning the attic which i presumed was a holding place for dolls who were "broken" but hadn't finished their contract but that makes no sense why not just let them go?

I'm starting to think that maybe the dollhouse don't want to risk the dolls recovering their memories or people probing in to where their loved one has been for five years and suddenly appearing surly that's more suspisious than someone with a trouled life just disapearing

i think mellie was a one off or maybe they just imprinted her again for pauls benefit and sent her to another dollhouse for paul's

I'm kinda torn about this. On one hand I don't see dolls getting released as being likely, it seems not only like a pointless risk of being caught, but a pointless loss of resources. If you have a doll doing well, making you a lot of money why release when they'll never know anyway? It just seems too nice for something with the high levels of shadiness the Dollhouse has. On the other hand, Adelle's reactions in Epitaph one suggest she was outraged at the idea of someone not being given their body back, so if they never freed anyone why would she react like that?
I think maybe the attic is for a mix of dolls who have gone wrong, so they are afraid of freeing them because their brains have been damaged/messed with, and people or employees who have crossed them.
I can't make up my mind, but I stubbornly refuse to add 'I'm not sure' as an option. I'm gonna come back when I'm less sleepy and make a more brain informed guess then :)

I agree about Mellie, if I had to guess I'd say Madeline is either another sleeper imprint or she was packed off to another Dollhouse.

I don't think the attic would be large enough to hold every used doll, just for those who'd been broken and for enemies of the Dollhouse. Also, if they weren't going to release old dolls, why put them in the attic instead of continuing to use them as a doll?