You folks have got my head spinning on this one.
First, and perhaps Per can comment with some authority on this, B-12 injected and B-12 orally are very different in humans, and I wonder if the same issue of absorption occurs in cats. (Per?) In humans with pernicious anemia, malabsorption of B-12 orally is corrected with B-12 injection. (I really simplified this issue.)
Next, and again maybe Per can comment, if the oral route is fine for appetite stimulation, would it be acceptable to take some of the 1/10 tablet of B-complex which is ground up, and sprinkle it on food directly? Would the taste discourage eating? How about brewers yeast, which is high in the B vitamins. (I'm trying to cut down on the extra work Scorpius is doing with the dilutions.)
Lastly, I feed my SV's 4 times daily with wet food and they have dry food always available. They eat what they want. They are not fat. They are active. They are healthy. They look good. Am I lucky? Are that many of you having challenges with your cats and their diet?