This is the final summary of the debate I had with the Reformed Constitutionalist. One of the things he pointed out, aside from ridiculing the idea that the Celts were Orthodox, is that the Western church in the early middle ages meditated on the Fathers, too. They maintained the faith, too. It wasn’t just the Eastern guys that had a monopoly on the Fathers. The West had it, too, so that means that the West is just as good as the East, right?
My original answer (which is in email, which I will not divulge here) was less-than-adequate, now that I reflect up on it. I had originally asked him to prove such sources, aside from a vague reference from the well-written, but admittedly non-scholarly How the Irish Saved Civilization. None was forthcoming.
That being said, I realized this week that the best refutation of his argument was simply to agree with his premise! As I mentioned earlier, there was yet no schism in Europe concerning the Faith. In fact, we shouldn’t even speak of “East” and “West,” for all was Orthodox (of varying flavors, it should be admitted). From a strictly legal point of view, which even Roman Catholics will admit, both East and West were one church, for the patriarchates were all in communion with each other (including Rome).
In fact, the Western Rite liturgy is Orthodox. Secondly, following this point and in contradiction to many overly-zealous Eastern Orthodox apologists, the Western middle ages knew no hard and fast division between the liturgy of the Western world and the Eastern world. Different appearances at times, but the essence was the same. St Olga of Russia sought after German liturgists. St Tikhon of Russia even updated and corrected the Anglican Book of Church Order (which I am not endorsing, but simply noting).
Both Eastern and Western liturgies had the epiclesis, the nunc dimitis, the te deums, etc. There are notable differences, to be sure, but they are of degree, not kind.
So the final answer: I agree with my interlocutor, but I also know that there was no division, whether spiritually or legally.