So why should we believe the event in Acts 15 actually took place, or if it did, how can we place it chronologically with any certainty ?
I agree with your conclusion Phizzy.
It certainly is doubtful that the Jerusalem meeting in Acts 15 took place.
According to Acts 15:2, 12, 22 - 26, 35, Paul was at the meeting in Jerusalem at which the Holy Spirit issued the requirement to abstain from foods offered to idols etc. However, in Paul's writings in Galatians etc, there is no mention of any such meeting taking place which Paul attended.
It seems the Jewish Christians were just trying to find a way to legitimize the continued observance of the law of Moses - they went so far as to declare that their decree in Acts 15 had the backing of the Holy Spirit.
The Jewish Christians however, had reason to insist on the continued observance of the Law of Moses:
They couldn't see how a covenant given by their perfect God Yahweh could be flawed as claimed by Paul. Paul stated that there was need for a second covenant to replace the first covenant because the first covenant was flawed:
Hebrews 8:7 (NIV):
"For if there had been nothing wrong with that first covenant, no place would have been sought for another."
Apparently it was difficult for the Jewish Christians to understand how the first covenant could have been flawed if Yahweh, the perfect one, had given it to the Hebrews.
This is one of the reasons why the Jews to this day have not accepted the New Testament or Christianity - how could a perfect God have given them a flawed covenant which needs to be replaced?