The only supplement that will really increase appetite is nutrabol. Nutrabol is an HGH analogue that signal the pituitary gland to increase HGH output. One of it's side effects (if you can call it that) is an increase in ghrelin, which is your body's hunger hormone. You can order it from sarms1.com. Here is an article on it for more information: http://www.evolutionary.org/nutrobal-mk-677-ibutamoren
Besides that, base your diet around foods that you grew up on and enjoy eating. Stick with easy digesting food sources like rice, potatoes, tuna, egg whites etc. If you are having trouble eating enough, you could try blending your food.
Not so I've noticed. Both my mum and my GF get B12 injections. Mum is 73 and my GF a year or two older than me (I'm 53). I've never heard either of them say they ate more on it
It'll help with energy and it's overall good for you. Do your research on b12 though. There are 2 different kinds. Methyl and Cyano Pretty obvious to me which to get.