"factual biological evidence that b12 can only be assimilated/absorbed in certain areas of the intestines."
It can be absorbed most likely any where along the path from mouth to exit...
In both cases keep in mind that skin, both internal and external is made of epithelial cells... if one can absorb transdermally, one can absorb anywhere throughout the digestive process... if conditions are the way they are supposed to be.
Other studies have shown decreased B12 absorption with the removal of a part or all of the small intestines ileum; also with impairment of pancreatic function... there is additional information on the absorption of B12 in the duodenum and jejunum of the small intestines... liver function also plays a roll.
My guess is that if one is "deficient" and/or not absorbing and utilizing dietary sources of B12, there is a digestive impairment, or a bacterial/yeast imbalance of some kind.