You have some truth here regarding the last 12 verses. Would YOU like to be the person who makes the decision of whether it was added or removed from the original manuscript?
I certainly don't see theology that is usually repeated somewhere else in the Bible discuss snake handling and drinking poisons. Plus, it does not make a lot of sense as an ending for Mark. I personally think we may have lost a true ending of Mark, but if we did it was God's will.
Though we have no accounts of Christians drinking poison, we do have the account of Paul being bitten by a venomous viper and not being harmed (Acts 28:3-6).
This supports the KJB text.