Yes, pernicious anemia can caused due to vitamin B12 deficieny make one susceptible to stomach or intestinal cancer.
Yes, they can be taken together there were no interactions found between the two.
Anything more than 900 is above normal. 1868 is very high and it should be dealt immediately to prevent kidney and liver conditions. You are recommended to take expert advise from a doctor to prevent chronic ailments.
You are recommended to check with a doctor. In addition, alcohol research claims that drinking alcohol in moderation helps in treating high vitamin B12 in people.
If you are experiencing unexplained tiredness along with vomitting and loose motions then you might have B12 deficiency. Get a test done to know the exact cause.