All of this has ramifications for modern times.
Perhaps, but nothing at all like you are suggesting.
First, I think that you read too much into Brest, and read it far too anachronistically. Whatever today’s American neo-orthodox think about ecclesiology - Orthodox or Catholic - has precious little to do with the church in 1600 or even a half millenium before that - notwithstanding the urge the some Orthodox have to ignore the facts of their own history,
More importantly, it should be clear that whatever ecclesiology was plausibly on the peoples minds from 1600-1990, it absolutely did not include the idea of churches moving into and independently establishing parishes - let alone eparchies - within the canonical territory of other bishops. This was not done.
Of course we never had, or likely anticipated, the pattern of enormous immigration that we saw at the turn of the century into the US. It could not reasonably be assumed that the
modus vivendi developed in Brest and used in Uzhhorod would simply transfer to the US without some difficult work and settling in. The fact that this settling in took a century - a century that also saw the development of a new perspective of sui juris churches - may be considered painfully slow, but it was actually rather fast as these things go.
And perhaps this history does have a lesson for today. I suppose that we can all agree that no Orthodox jurisdiction would ever defer to another. We see this attitude, which has led to the flaunting of tradition and canons, in the weird ecclesiological structure of Orthodox America. I suppose some Orthodox think that this is a great and virtuous ecclesiology, but I know that others - clergy and hierarchs - see it as evil and wish for some primatial authority that could adjudicate these problems.
So, is the history of the The Greek Catholic churches in America honestly an impediment to re-union? Hardly! It is the very existence of our churches is the major stumbling block - at least in major centers of Orthodoxy. And if the history were important, the lesson to any objective observer is this: with time, even if not without trouble, a better and highly workable framework was worked out in the mere course of a century. How long will we continue to wait for the Orthodox to figure it out? Perhaps if Orthodox spend more time working on their own present problems, than musing about our historical ones, they would make more progress.