Following OT principles vs. legal obligation

Questions about whether anyone who tithes is not a Christian and is accursed since Paul says that if you obey one part of the Mosaic Law you’re obligated to obey all of it, and the claim that tithing preceded the Law and therefore remains a principle for the church today.  

• Since Paul says that if you obey one part of the Law to please God you’re obligated to obey all of it, does that mean that anyone who tithes is not a Christian and is accursed?
• How would you respond to the claim that tithing preceded the Law of Moses, as evidenced by Abraham giving a tenth to Melchizedek in Genesis 14, and therefore remains a principle for the church today?