That's a tremendous answer, not least because it raises as many questions as it suggests answers to the ones asked thus far.
If we were to take St. Irenaeus' suggestion about the Apostolicity of Sees, then we should, as you say, Fr. Gregory quickly find ourselves mentioning his own citation of Rome as the best known and best-attested example. Rome itself would say that those things which to which the Orthodox object as novelties are, in fact, legitimate developments of the Faith once given; legitimised, of course, by one of those developing understandings - the position of the Bishop of Rome.
The people do, indeed, have a great part of play in saying what is 'Catholic and universal', but how that part is to be played is a vexed question. The largest Christian Church in the world today, as for a long time, is the Roman Catholic Church, and in so far as we can determined what universal might mean when it is at home, that Church is certainly more ubiquitous than the Orthodox Church. Phyletism has certainly been defined as a heresy by the EOs, but whether the OOs have done so I don't know. Either way, it tends to lead to mistaking ethnic practice for orthodox praxis; assuming, of course, that we can define the latter, and define who the 'we' who does the defining might be.
Here, for whatever limitations it has, the RCC has clarity where we have what might be called fudge majeure. The teaching authority for the RCC is not quite just the Pope; for us it is whom? One might, indeed say that it is not our part to say who is and is not a member of the Catholic and universal Church, simply to assert we are, which brings us back, perhaps inevitably, to the initial question of who is to say, to which the only real answer is 'God'.
In this is love, not that we loved God, but that He loved us and sent His Son to be the propitiation for our sins. (1 John 4:10)