Havoc, in Deuteronomy, God gave men authority to interpret the Law. Jesus himself acknowledged the authority of the Pharisees to interpret the Law.
Please keep in mind too what I have pointed out previously: that the Pharisees were not a monolithic group. The Shammaites were much more 'rigid' and 'legalistic', and likely were the group that drew Jesus's ire. On the other hand, Jesus had a great deal in common with the Hillelites.
You're helping me illustrate a point. I hope you know that.
The Pharisees were given their authority by God. It was also removed by God via the Messiah. The messiah is now the interpreter and judge of the Law and the Commandments. He is the King of Isreal and of the Congregation. As such, the priesthood answers to Him and derives from Him. And the bible says we all who believe are now priests. The spirit of God is Given us to interpret for us directly. If we get it wrong, it is our fault because God made all the resources directly available to each of us. There is no excuse for failure. And the only reason for failure is disobedience to God. God knew what He was doing.
Jesus rebuked the Pharisees and denounced their methods and their traditions; but, you are correct that he did say they should be obeyed at the time because they still held places of authority. Upon the death and resurrection, that authority was revoked. The priesthood of old was now useless. And anyone now claiming a priestly authority has none. Teachers and Preachers are servants to be heeded. We ignore those who teach correctly to our own peril. In heeding false teachers we do that to our own peril as well.