Have you tried removing the rooster? He is not necessary once the hen is laying eggs. A variety of nesting places might help induce broodiness. My hens prefer a box that is secluded where they can feel secure. Even a sporadic layer can go broody. My first serama hen took close to three weeks to lay six eggs and then go broody.
Sorry.
I understand all about frustration and birds; really hard to take at times. My only other thought is to give more room. I have not been following this thread very closely so I do not know if you have a coop or these are house pets/or something else. Some hens just will not brood if they are in smaller surroundings. My silkies would brood if kept in a cardboard box. Other hens I've had required more room.
I have an incubator, but I only use it if I have no other choice. I keep plenty of proven broodies and it's pretty rare for me not to have hens brooding.
If the hen doesn't brood either conditions are not right or she is not parent material. The hen would not know that her eggs wouldn't hatch. Hens, where there is no rooster, will brood.
Try to make the pen as big as you can with more than one nest site to choose from. I try to position the nest boxes so the front is pointed away from the light so the lighting inside the box is dim. That's a thought-do you use boxes with a roof? If not, give that a try. Most, not all, of my hens prefer the nest boxes that have a top and solid sides and back.
