I wouldn’t rely on DE to kill lice. I would use either Permethrin garden dust or the permethrin spray on the birds. If you buy a small bottle of Permethrin 10 concentrate, that can be mixed with water 1 tsp per quart in a spray bottle for chickens, and in a garden sprayer for treating the coop...
Has Basil molted this year, or is she looking like she may be? If that isn’t her problem, she might be showing signs of a reproductive issue, worms, or something else. It can be hard to figure it out. Sometime we don’t until a necropsy can be done if we lose them. I would worm her with Valbazen...