Hi there! *waves hello* Welcome to RFT! :)
I'll try to answer some of your questions. In regard to B12, supplement. Also, some people incorporate nutritional yeast into their diet to help with this as well. It's not raw, so it's a personal decision.
As for your concern about "too much" of a food, I say (and my guess is many would agree) listen to your body. I've found that since beginning on my raw food journey that my body tells me what it needs and when. I was consuming mass amounts of avocado, I mean MASS amounts...but once my body felt like it had enough the desire curbed itself accordingly. I still eat them but not like they were going extinct like before. Then at one point, I was addicted to bananas...I've found that my body will let me know when it's had enough and is ready to balance things out. My non-scientific guess is that it's my body's way of balancing out what it was missing as a result of my previous SAD diet.
I can't answer the question/concerns regarding not bulking up, etc. So I'll leave that to someone else.
Best wishes on your journey!!
"Make the most of yourself, for that is all there is of you." ~Ralph Waldo Emerson
"Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has." ~Margaret Mead