I'll take a stab at it, Sebastian.
For me orthodoxy has to mean just that, right thinking. That means that one, whether that is an individual believer or
a group of believers, or
the group of believers we call the Church, that one, I say, has to be able to judge whether what he thinks is right by some objective standard outside of himself. It's the ability to see the marker in the coastless sea to guide your direction, whatever that marker may be. You need to be able to navigate your direction so that you are heading toward your destination. Not like Colombus' first trip, where he just went in the general direction, but like a modern-day ocean liner headed straight for her port even though the captain does not yet see that port from the helm. Being able to direct yourself toward the goal of true thinking is what I call orthodoxy.
It is not decided on arbitrarily by anyone, It is recognized by believers. It is officially recognized by the Church, as those believers who meet in holy convocation, pray to God, and decide by the direction of the Holy Spirit, and give thanks for His guidance.
Heresy is about some view or opinion that sticks out, that goes against the unity of the group on an agreed or constitutional matter. But in the church it is not heresy to be the lone voice appealing to truth, while the church goes off on her own course. In that case the church is going into heresy, and not the lone believer. So heresy has to be defined by some referent above the church. And that has to be the Bible as interpreted by itself, not the church.
WCF, I, iv: The authority of the Holy Scripture, for which it ought to be believed, and obeyed, dependeth not upon the testimony of any man, or church; but wholly upon God (who is truth itself) the author thereof: and therefore it is to be received, because it is the Word of God.[9]
9. II Peter 1:19-20; II Tim. 3:16; I John 5:9; I Thess. 2:13; Rev. 1:1-2
Pelagianism and Socinianism are not heresies because the Church said so, but is rather the case that the Church said so because they
are heresies. The Bible puts adherents of such views at odds within the Church's agreed position, and that makes them heresies. They are proven to be so by Scripture's clear revelation, and not by Church tradition, or the Church's own standard in addition to or beside the Bible.
In summary, if right thinking depended at all upon man or the Church to put context to the truths of the Bible, then there really is no such thing as right thinking, and orthodoxy is really only a pipe dream, it does not really exist for man. But the very fact that we have a word such as orthodoxy, that we have an aim for objective truth, that we have a reference point in mind to guide us toward that aim, means that orthodoxy exists, and that therefore an objective reference point exists. Or, from the other direction, the fact that we can say with certainty that some view is wrong means that we have some reference point of truth from which to assert error.
This, I found, is quite the same as trying to learn perfect pitch. Without a piano in the room, hum for me a D. Can you do it? If you can't, then you don't have perfect pitch. You can learn it, but it is hard to do. I'm getting a lot closer than I used to be. I used to have no clue at all as to how to go about it. Now I do. I don't have perfect pitch, but I'm a lot closer than I used to be to being able to give you a D. It's a whole note below E, and E is the one I can get closest to. But what I'm working on here is a reference point for pitch that a piano has to conform to as well, not just my conforming to the piano; and being able to reach it even if a piano isn't handy. It's kind of like a musical orthodoxy.
That's my opening thoughts on this.