All you can do is keep giving her vitamins, you’re doing great.
It could be a vitamin deficiency alone that’s causing these symptoms but she could have caught an illness as a result of that or she may have caught a virus or even botulism.
You could try to get her seen by an avian vet but...
Im not sure what’s available to you in the U.K but if you can get some ivermectin horse paste that will rid her of the insects.
Also a liquid multivitamin may be needed.