Rev. John Chryssavgis, archdeacon and theological adviser to Patriarch Bartholomew of Constantinople, in an interview with Crux, says it won't affect it at all:
If one or more churches doesn’t attend, or withdraws during the council, or is not present and doesn’t vote, all the decisions made will still hold and be binding for all Orthodox churches. A Great Council is above and beyond any individual church council or synod … and it remains such even without the participation of one or more church.
However, further on in the same interview, Rev. Chryssavgis says this:
It’s also important to remember that the time after the council will be just as crucial as the event itself, because it’s the period of reception. No rule or structure in the Orthodox Church comes from the top down. It’s the conscience of the faithful, the Church itself at large, which is the ultimate protector and guarantor of Orthodox truth and doctrine.
So if the Orthodox faithful, over time, end up not agreeing with the conclusions of the Council, then it may end up not having any authority.
So how authoritative is this Council? The real answer is . . . Who knows?
For more, see here.