What Old Testament messianic prophecies tell us that the Messiah would set aside large parts of the Law and the Prophets?

Christian preachers today teach that we are under the new dispensation. Good Christians are not only allowed but required to disobey the commandments regarding the Sabbath day. They are allowed to eat pork and shellfish, wear mixed fibers (PremaPress), plant mixed seeds (flower gardens), worship other gods (money), and forget about burnt offerings. Women can lose their virginity before marriage. Sons can be disobedient. It’s frowned upon, but not a capital offense. People can commit adultery. Even homosexuals escape death by stoning. All these crimes were punishable by stoning under the Law and the Prophets. Somehow, Christian theologians have deemed all the laws except those against adultery and homosexuality as now null and void. And even homosexuality and adultery are no longer capital offenses. Indeed, most Christian nations view death by stoning as barbaric.

How do we reconcile such changes with what Jesus is quoted as saying in Mathew 5? “17—Think not that I am come to destroy the law, or the prophets: I am not come to destroy, but to fulfil. 18—For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. 19—Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven. 20—For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.”

Is there anything in the Torah that foretells later day prophets further perfecting the supposed changes Jesus implemented. For instance, does the Torah give credence to Mohammad, Joseph Smith, Mary Baker Eddy, Sun Myung Moon and such later day “prophets”? Why would a God who is omniscient and omnipotent need to spend thousands of years getting his message just right?