Lucian,
I'm not sure that there is a solution to your epistemological question.
Not one that can be solved without apparently circular reasoning--when viewed from a human perspective--anyway. It is true that we are human, fallible, biased. As humans, due to "nurture" or "nature" or both, we have certain tendencies, views, and ways of looking at things. There is an ultimate Truth, but since we are sinners we do not have a perfect vision of that ultimate truth: only the Saints know Christ the incarnate Truth as He really is (insofar as any man can comprehend the God-man).
Seemingly, I've pondered over the same things you have. I remember having a debate with a Protestant, when I had just decided to become Orthodox. He put up the Bible (ie. his interpretation of the Bible) as his authority; I put up the Church (ie. my understanding of the Church's understanding of God's revelation) as my authority. As you have repeatedly said on this thread, though: in the end, it all came back to us as individuals, since we were the fallible interpreters trying to understand. As you said, the Church is an infallible interpreter, but what does that mean when we fallible ones must try to understand (ie. interpret) what the infallible Church (infallible because Christ is the head) says?
I believe that there is an answer, from an Orthodox perspective. If you have access to Orthodox Faith And Life in Christ by Saint Justin Popovich, perhaps you could read his essay on the epistemology of St. Isaac the Syrian? I believe that St. Justin really brings to light the truths that Orthodoxy teaches about these issues that you are bringing up. I'm not sure how well I can sum those truths up, but for those without the book (or just for discussion's sake) I'll try. Please tell me where you think I get this wrong--or where this process of thought goes wrong.
Truth is linked with holiness (St. Justin also goes over this a good bit in his essay on Theanthropic and Ecumenistic Education). The Saints are not infallible per se, but their lives are sort of "living dogmatics," and because of their contact with God's grace attain certain abilities and virtues we sinners don't have. One of those abilities is normally the ability to discern truth from falsehood much more easily. In a word, the holier you are, the better you are at detecting truth and falsehood. Everything plays into this: outward asceticism, love, faith, prayer, meekness, etc.
Also, nothing is revealed to us except that which God decides should be revealed to us (and if God does not want to know it, even if he allows us to read or hear something, then we will forget it). Jesus said of the Pharisees "had I not come, they would not have sin, but now that I have come they can see their sin," meaning that God decided when, and where, and by whom the Pharisees in that passage would come to the knowledge of the truth. And God decides for each of us what "measure" of truth we are allowed at any given time. That God will allow us enough truth (e.g., to avoid heresy) we simply have to take by faith.
So we say to ourselves, "How do I know whether what I believe now is the God-revealed truth, or just my own incorrect interpretation?" Obviously we should study and seek to make sure that we are not incorrect. In the end, though, the answer is that it is only in pursuing a pious and God-pleasing life that we can be sure that we are on the right path. Looking at other apparently pious people does not always work, since sometimes the apparently virtuous acts of people (even heretics) can outshine those of the Orthodox. As our Lord says, many will say to him on the last day, "Lord Lord": but doing "great acts" is not proof that people know Christ our God.
It all comes around to living meekly, humbly, lovingly, and sincerely. Our faith must be sincere, or the longest beard, the most prostrations, and indeed even raising someone from the dead, will do no good. Satan keeps vigil all night, knows the Scriptures, fasts constantly, etc.: these things mean nothing apart from the proper mindset; it is only in humility and sincerety that our own deeds are acceptable sacrifices before God. And it is most times only (paradoxically) in humility that God will decide to grant us great understanding. God allows the "wise of this world" to keep their great wisdom, but such wisdom is foolishness to God; while God allows the simple fisherman, tax collector, and so forth learn the secrets that the greatest Old Testament Saints such as Moses, David, and so forth only caught glimpses of.
And Jesus said, "and you shall do even greater deeds," and "even the least in the Kingdom of Heaven is greater than John the Baptist". But "The first shall be last" and "he who wishes to be great must serve". And so, while it probably isn't a very popular response, I think that the key to solving the epistemological dilemna is choosing salvation, which is a steep and rugged ascent. It is a difficult journey, one in which we get attacked during every day. You are right, I think, when you lament how hard it is to know what to do, when it all comes down to the interpretations of we petty sinners. But the solution is for us not to trust us, but Christ in us. Of course, Protestants and Catholics could say that they do exactly that, but I can't argue with them.
All I can say is what I--as an admittedly horrible, fallible sinner--believe: that the Scripture, the Fathers, and the Church (authorities accepted by both you and I, even if we interpret them differently) all say that one must pursue holiness in meekness and sincerity in order to come to the knowledge of the truth. "You do not have because you do not ask; you do not get what you ask for because you ask wrongly [ie. insincerely]". The first step--by God's grace--is coming to the point where you wish to find the truth: you then ask God for the truth. The second step--by God's grace--is working out your own salvation with fear and trembling, seeking holiness and peace with all men, and so forth.
So how do we know whether such and such a jurisdiction has grace, or whether such and such a jurisdiction are wrong about such and such an issue? We can only trust outside sources (e.g., what a saint says) to a certain extent, since we must allow for the fact that even saints make mistakes, and especially that we are very fallible interpreters. The only sure path is to seek holiness, God's cleansing grace, and then trust in God's love, mercy, and providence: that we will be judged according to our place in life and what we did with what we were given.