Melchizedek is a type of Christ; the Scripture does not say that he is Christ. Beginning your post with "if people would simply believe the scriptures" is humorous given this is what you write immediately after.
Nor is there anything to declare an end to stoning, but we don't do that today. The absence of a specific abrogation does not invalidate the overall abrogation of the Law as explained in Romans, Galatians and Hebrews. There were examples of tithing before the Law, but there was no clear commandment (as there was with circumcision). Abraham tithed war spoils and kept nothing; Jacob bargained with God for abundant blessing and there is no record that he gave tithes of anything. In Malachi, God was speaking to priests, not to the people... read the context carefully.
As to the rest of your post, don't make the mistake of assuming that people who don't "tithe" (as you understand it) don't give to support the work of ministry... even generously.