What is the white material in her beak—mucus or?? Is she eating today? You might consider worming her today with SafeGuard liquid goat wormer 10% fenbendazole 0.23 ml per pound of weight given orally for 5 straight days, and starting some Corid in her water in case of possible coccidiosis...
Here is a good link with examples of chick slings and chairs, and the basic design is on page 2:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
She looks like her beak was trimmed, which is a procedure some hatcheries do to prevent them from pecking each other. I would moisten a small amount of her feed daily to see if that is easier for her to eat.
I would use B complex 1/2 tablet daily. You could also in addition try some human vitamin E 400 IU daily which is used in combination with a small amount of egg for selenium and the B complex for neurological issues. I hope that she improves. Sometimes we do all we can do, and sometimes do not...
It looks more like your hen has some pecking injuries or has run into some wire fencing with her comb. That is not blackhead, which is a disease more common in turkeys.
If she has neurological issues or an injury to her back, I don’t know that a hobble would work, but use your own judgement. Here are some good ideas for a chickens sling in posts 5, and 11-14:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
One of these might...
Casportpony has a lot of threads and expertise on tube feeding. I would say that as long as she is gaining some weight every day or two, you are moving in the right direction. I posted one of her ps in post 4, and here is another one with videos and and a lot of info...
Mercks vet manual says that 20 mg (200 mcg) daily is enough, 10 mg for chicks. But riboflavin and B complex is hard to overdose, since the excess is excreted by the kidneys.
https://www.thepoultrysite.com/publications/diseases-of-poultry/217/vitamin-b2-deficiency...
Many people treat their own chickens and remove any yellow plaques blocking the throat and beak. It does tend to cause bleeding, so beware. Many people cull chickens with canker, since it is contagious to other chickens and birds. I have never treated it, but I don’t believe that it is ever...