You've obviously done a bit of research. Have you ever seen the vitamin B12 link referenced in other studies? Are you taking supplements or did you get it tested?

As far as hair graying is concerned, I don't think that B12 does much unless you're seriously deficient in it.

There's some research in B-vitamins and PABA for hair repigmentation, reviewed (among other things) here: https://pmc.ncbi.nlm.nih.gov/articles/PMC6995950/

But the B-vitamin in question is calcium pantothenate (B5), and that B5/PABA research is very old -- from the 1940s and 1950s. The results were also hardly anything to write home about; a very mild effect at best.

I don't think that there is a reliable chemical agent to reverse hair graying at the moment, but it seems as though it's become possible in theory to target those mechanisms which appear to be responsible...