Can Magnesium Bicarbonate be absorbed in the mouth?"
Magnesium Bicarbonate occurs naturally in some mineral waters.
Technically, the human mouth do not absorbed anything. The human mouth is just a first line of defense mechanism against external microbial (In terms of saliva). If in terms of physical aspect, it just helps you to reduce food size and shape it for consumption.
Real absorption do not happen in the stomach neither. Stomach is where acids are used to break down your food and subject to higher lethal effect to kill microbial. The absorption only begins in the small and large intestine. (This is why drug delivery is such a hot topic in research labs).
To answer your question, yes. We need magnesium to survive and so we do absorb magnesium. In fact we can't live without magnesium, we need it for certain vital functions for our body to be normal. (Only in small magnesium amount, we don't need a lot of it).
As for bicarbonate, our body do not need it. In small amount, it is not toxic to our human body but in high volume, this would be a serious issue. When we breath, on cellular level, gas exchange occurs. Oxygen is supplied into our healthy cells while carbon dioxide is removed from our cells into our blood.
This is where it gets tricky, we do not store carbon dioxide in the form of gas when removing it via breathing out. We actually store carbon dioxide in the form of Bicarbonate in the plasma of our blood (Do note that its plasma, not RBC). When it is transported to our lung, this bicarbonate will be converted to CO2 and removed as we breath out. Hence, we do not absorb bicarbonate unless it reacted with some other elements or salt to produce some byproduct that we could use.
I hope you find this useful.