Am I really supposed to believe that the true fullness of the Christian faith is to be found only in scattered communities in Egypt, Armenia, parts of Syria and India (Plus their respective Diaspora's around the world)?
Not exactly. You're overlooking the Ethiopians and Eritreans. Also, the British Orthodox are part of the COC, but I don't think it would really be accurate to call them an Egyptian diaspora. But if the question is modified to include those, I would actually be comfortable saying yes. I do believe in the infallibility and indefectibility of the Church of Christ, however, I tend to believe something along the lines of what William of Ockham (if you want to read more about that: http://www.humanities.mq.edu.au/Ockham/winf.html
) whereby this infallibility and indefectibility can very well be satisfied by the continuation in purity of the most obscure section of Christendom. Thus, I think that Christian truth could be as obscure as the OO churches, or perhaps it could hypothetically be even more obscure than that. For instance, it could very well get down to one particular church like the ACE. Or it could even be down to a handful of bishops and their flocks. I thank God that the situation is not that
The largest group of OO's seem to be the Armenians
No, it's the Ethiopians.
who, from my personal experience are very clannish and not so welcoming of outsiders whom they seem to feel would pollute their already diminishing bloodline.
Hmmm. I have had a very different experience of the Armenians around here.
The Copts though are very friendly and, from what I've heard open to missionary work amongst their fellow Moslem Egyptians.
We have had very different experiences of these two groups. The Copts around here seem very insular and uninterested in outsiders.
It just doesn't seem possible to me that God Almighty would have let his one true Church become so diminished in the way that the OO's are presently
You're free to think that. But I think all that is logically required by the promises of Christ is that there will always be a flock (I don't know how many bishops that would require at the bare minimum, 1? 2? 3?) with the foundational faith and Sacraments. And seeing as how the distinguishing differences between EO and OO could very well touch on compromise of Christological orthodoxy, I think it would be best to not think beyond what is logically required in terms of the size of the Church lest we be led to trespass upon what is required in terms of dogmatic orthodoxy.
(Was their ever a time when they were the majority faith or, at least had large numbers of believers amongst Christians?).
They do now. From what I can tell, they're right now the 5th largest international community of Christians by number of believers (estimated at 75 million), after Catholics, EO, Pentecostals, and Anglicans. At certain times in the past they probably possessed a larger chunk of Christendom.