Or see if they like wet food. Most chickens like it enough to come running when they see you bring it.
So serving wet food inside the coop in the evening could lure them all in without needing to take away food during the day.
If you distribute the wet food in several dishes, or spread it out...
It sounds like you have been very thorough in addressing the points we could come up with, so probably the only thing left is to break the pullets' habit of roosting in the trees by locking them in for a while :(
Your idea of a curtain to divide the roost might help too, but probably isn't...
How cold does it get?
A chicken roosting in a nice dense evergreen will probably do BETTER than a chicken in many styles of chicken coops. She's got protection from wind, rain, and snow, and I'm sure the tree has great ventilation!
I agree that is a concern (although she obviously hasn't been...