If Papadopoulos was so central, why did the FBI not interview him until late January 2017?

It seems as though the Papadopoulos origin was drummed up once the Carter Page origin story fell apart.

All media sources were reporting on the dossier as the origin until it fell apart.

What does the date of the interview have to do with whether or not he was central?

Lack of evidence is not a conclusion. It's what you'd expect if there were successful obstruction of justice. Even Barr's summary quoted that the report "does not conclude that the President committed a crime, it also does not exonerate him."