Sorry if I sound argumentative, gem, but I do see problems with this as it pertains to mental health. I also apologize if I sound snobbish about this - I just feel very strongly about the ethical practice of psychology and see every day how the field is easily misused to malign, discriminate, and harm.
I feel much the same way about "experts" who go on television to come up with some off-the-cuff diagnosis of someone they've never met; it is the worst kind of quackery. We can certainly go back through historical records to say - in all fairness - that Mary Lincoln dealt with mood changes and was often subject to angry outbursts. It's documented by several people who were there. But assuming we know the cause of these is irresponsible. Maybe she had a brain tumor. Maybe she did have this vitamin deficiency. Maybe she was tired of people on her case all the time. Maybe she felt like she was going "crazy" after watching her husband get shot in the head at a play she wanted to go to. I'm not an advocate for stifling inquiry or suggesting that we can't ask about the cause, but I ask you to consider that assuming pathology without sufficient evidence is harmful. This, in my opinion, is especially true for women of the time (for some really upsetting reading, try looking over Freud's view of classical hysteria), who could easily be pathologized rather than supported as they dealt with tremendous change and loss.
Please know I don't intend any of this as a personal attack towards you, gem. I've been in the field long enough, however, to know how sausage is made (so to speak), and how ugly psychology can be if we allow it. As I said in an earlier post, I feel for Mary Lincoln and countless others that suffered tremendous pain, but are being treated as mentally ill in some modern-day exercise in psychological sadism, simply because we can. I don't think it's a good use of historical inquiry, and it's a dangerous use of psychobiography.
Just my thoughts,
Adam