Just as an aside, using the last verses of
Mark 16 are questionable to say the least. There is a lot of debate as to whether those verses, including vs 16 which I really like, even belong in scripture as it seems fair to say they were added much later.
You don't need
Mark 16:16 to prove the necessity of baptism for salvation. The Epistles do a fine job of showing that the ONLY way we get IN Christ is Baptism.