Don't worry. You are doing fantastic. Hang in there. Everyone here on BYC will do their best to help you. There is so much information for you to read here that in no time you will be an expert poultry keeper.
Many of us do not have an avian vet at their disposal. I'm one of them also. Sometimes the best you can do is read all you can and then give it your best shot.
You can buy Corid pretty reasonably at your local farm store. Tractor Supply, Orschelns, Farm and Fleet, etc. You can also find it on line at Valleyvet.com, Jefferspet.com and believe it or not, you can find it on Ebay at a reasonable price. Don't just treat the hen with problems, treat them all.
I agree. When you don't know where to start, what is really wrong or what to do about whatever is wrong, the best place to start is worming. You have covered the possible vitamin deficiency.
It doesn't look in your picture as if she is sitting fluffed up. Is she doing that at all? Is her head pulled down so she doesn't look like she has a neck? If so I would suspect an infection of some sort. Is her comb and wattles red or pale? If pale, it can be a sign of illness or parasites.
She is eating? Are you giving her extra protein? Some chopped hardboiled egg or tuna? I even give mine left over chopped up meat from dinner.
Maybe
@casportpony, @Wyorp Rock and
@oldhenlikesdogs will join the discussion and offer some new suggestions for you.
You are doing a great job. Hope your hen gets to feeling better.
