Two questions:

If they are used to their sleeping area, they should return there each night. Of course there are sometimes exceptions.

Most chicks will progress to the roost as a normal maturing, however, I have a cock, 2 years old, that has never flown to a roost. He`s the exception and I have for some time doubted his sanity.

Eighteen weeks is probably a good enough time, but if you have a problem, you might want to cover them until the pullets start getting pinkish faces, as a sign of maturing......Pop

 

My hens were slow to roost, too, and even now they prefer a platform to a roost, which I have given them.

 

My baby chickens were like that too when I got them settled outside. I left the light on day & night until they were around 3 full months old, this way gave them plenty of time to adjust to their new home & feel safe. When I did finally turn the light out for good their instinct kicked in and told them to get in and settled before dark & they are almost 4 months old and doing just fine!
Now for the nesting box question I am unsure how to answer that because mine are in a grow up pen without the nesting box's they will get moved to the adult pen at the end of June when they reach point of lay. There will be someone here who can answer!