I really do hope little Ruby can get better, she is such a sweetie. You have so many experts to help you too (not me, as I said before, I'm sheeple).
But I did have a much loved sheep with similar anaemia and debility etc, as I described before. My wee Bramble went through all kinds of tests at the University Vet School Farm Hospital which happily wasn't too far away. Have you specifically excluded or treated for (as an "in case" type of scenario), liver fluke, coccidiosis and mycoplasma?
I know nutrition is vital to get right and there are many experts here on the case for that with you, so I just wonder if she has actually got any infection which is stopping her getting better?
Might be worth asking the vet if they have excluded these sufficiently? I was told fecal samples can be unreliable in cocci (in sheep anyway) so it might be worth giving a coccidiostat, but flukes should be seen at this time of year, I believe, on stool samples. Mycoplasma may be easier treated with an appropriate antibiotic such as oxytetracycline or enrofloxacin rather than confirming it first on a blood test (and cheaper - check with the vet). There are probably (of course) many other infections that I am not aware of as well that it could be. Anaplasmosis someone mentioned - I know nothing about this as we don't get it in the UK, but maybe someone can advise on diagnosis and treatment (either here or hopefully a vet!). I am fairly sure that a fecal and a normal blood panel (other than anaemia, which means something is wrong) is not going to exclude all infections and "medical" causes. Also are you sure she hasn't been poisoned - for example brassica family vegetables/plants can cause anaemia too.
I'm not a vet - just speaking from a long and stressful sheep experience. I really feel for you, it is so hard when you feel so helpless.