I don't know how old your birds are or if they've been trained/raced before but it sounds like they have never learned to leave the basket and head straight for home!
when you get to the lib site, stand you baskets out of the car so the birds can have a good look around and get there bearings.
Also what is motivating them to go home?
I'm sure if you reported them to the unions and explained how you came by the birds the registered keepers would be happy for you to keep them and register them in your name.
You need to do this as currently they are not yours!
you list the ventilation you have at the front of the loft but what do you have at the rear and roof level?
If air can only go in through your vents where does the stale warm air exit? No good letting fresh in if stale can't get out!
Don't forget there is an ex-forces in charge of the RPRA who like his picture in the paper!
Any punlicity is good publicity and every opportunity should be grabbed!