I'd have the basement tested for any mold. That could be effecting the birds big time and it could effect you. This is the first thing that comes to mind for me.
Another thing, I don't agree with you selling the hens to stop breeding. Just don't give them boxes and you should be fine.
As far as the babies go, Might have been ideal to have homes lined up before you bred. I knew a few people who wanted babies even before i bred and i have people who would gladly go on a waiting list besides with all the troubles i've had this season i'm done for the year. Needs a break unless the conures give me anything.
You say Pets Plus has sickly birds and you knew this so why bother buying more birds from their? Why not advoid that store all together.
I personally think it would be in your best intrest to get at lease one of your birds to a avian vet to be tested. Lease this would ease your mind some. I've been down that road with my guys. $500 later i knew my flock was free of any illness
im not selling my hens im not gonna go through breaking up pairs as for mold there is none my old house had mold black mold and pets plus is were my sister got if from i dont buy my birds from there the other hen that died came from pets supplies plus there birds aren't sickly and for selling ive posted on craigslist but the closest to me is buffalo and thats 1 1/2 to get there theres not that many people in this town