It's not a matter of denying anything. Notice to whom He is speaking when He speaks. All through His ministry, through His Passion and Resurrection. The Day of Pentecost.
It's always to some or all of the Apostles. Paul's epistles are addressed to churches. The common thread is that He deals with groups, not individuals. The pattern is OT and confirmed by Paul to the church at Corinth..."at the mouth of two or three witnesses..."
The first council in Acts 15 has been discussed here many times. There was a pattern laid down. So there is certainly scripture to support the convening of the bishops to seek the Holy Spirit about disputed matters.
Transferring everything committed and promised to the Church to the individual is going to create confusion. Beyond puzzling over why Jesus did not tell the Apostles to write it all down, I ask what about the Christians who had no New Testament? Answer: They were to be in the Church and as Paul says over and over, act and believe like HE had done told them to act. and believe
When, exactly did the common practice of the early church, as shown above, get negated? By what authority?
Not the way the Catholic Church portrays it he isn't. He is addressing the ekklesia of believers. The assembly of those called out. Catholics will never understand what Christ and the apostles were talking about or who they were talking to unless they understand what the ekklesia really was.
>>The first council in Acts 15 has been discussed here many times. There was a pattern laid down.<<
No, there was not. And at that same meeting recorded in Acts 15 the statement was made that all believers had been given the Holy Spirit "just as they had" making "no distinction between us and them". Not once in all of the New Testament an assembly of those called out under any superior organizational hierarchy other than it's headship which is Christ. In Revelation Christ spoke to each of the assemblies as separate. He didn't speak to some hierarchical head "church" to straighten out each of it's subordinate branches.
>>Transferring everything committed and promised to the Church<<
There's that corrupted concept of what "ekklesia" means again.
>>Beyond puzzling over why Jesus did not tell the Apostles to write it all down<<
Actually it's beyond puzzling why people would follow an organization that doesn't follow the apostles admonition to not go beyond what is written.
>> act and believe like HE had done told them to act. and believe<<
And still Catholics follow an organization that cannot prove that what they teach is exactly what the apostles taught. So Catholics, rather than putting faith in the very apostles Jesus chose, put their faith men who claim to have "passed down" some unwritten story. That puts Catholics in the same position as Muslims and Mormons.
>>When, exactly did the common practice of the early church, as shown above, get negated?<<
It was never instituted in the first place so there was no "negating" done.