I doubt the B-12 disagrees with you. The dehydration you mentioned has nothing to do with water soluble vitamins. You are dehydrated because you are not drinking enough water. A head injury can change how things taste and alter appetites. You are likely doing something different with your diet.

There are many different versions of B-12. Get the methyl version in sublingual lozenges. Chew it and let it soak into your mouth.

You have been overly dependent on the doctors and others who have told you to keep working. As you say, your gut tells you to not work until you are recovered. You should trust your gut. The doctors do not have a clue what is going on at your job.

Take time off. Get some quiet rest. That means limited stimulation. Do light activities that do not require cognitive focus.

I don't understand why you have curtailed your non-work activities but are still plowing ahead at work and feeling lousy when you do. If you had made the same choices about work, you would be weeks ahead in your recovery.

Stop the work, not the B-12.

My best to you.