As far as an argument from authority, I wonder whence you dervive the statement "arguments from authority are weak." Perhaps in a political context, but in the Church we believe in the authority of the bishops. Of course we also believe in the laos (people of God) but many of the educated people of God fully support St. Vladimir's and its presentation of the Orthodox faith.
Arguments from authority are not necessarily "weak", simply "the weakest." Paradoxically, however, if the "authority" involved is sufficient, they can be anything but "weak" arguments. For example, if the authority is God, then it is a very strong argument (even if the logic involved is not apparent.)
However, it is not clear here that the proffessors of St.Vladimir are speaking with an authority greater than themselves. It's certainly not been established that the views that you asbcribe to them, are rooted in the tradition of the Church (whether it be the services, Fathers, or the Holy Scriptures.) Thus, in a case like this, simply citing "their" authority is a wasted excercise (and thus why in logic, arguments from authority are generally the "weakest" arguments.)
If an Arian in Germany didn't know that Arianism isn't wrong, and he was a born member of that Church, he is not a heretic. He is simply in error. Latins distinguish between material and formal heretics, the former being someone who believes the wrong thing and the later being someone who professes it. I believe the distinction is valid despite its source.
In that case, there are practically no heretics to speak of (and never have been) since even heresairchs (instigators of heresy) are usually quite convinced that they are correct.
If you had read my earlier response carefully, you would have noted that it was I
who made the very distinction you bring up (entertaining the notion of there being a difference between "formal" and "material" heresy), as well as mentioning possible culpability. However, that still leaves the party involved a "heretic". It seems to me that the real problem is that the term "heretic" is perceived as being uncouth, so the ecumenically inclined don't want to use it (even if it fits the bill.)
If you want to cite the Latin distinction between "formal" and "material" heretic, you have a problem; since a "material heretic" is still a heretic in old school Latin thought (thus why, before the Vatican started subscribing to ecumenism, RC moralists and theologians definatly considered Lutherans, Presbysterians, etc., to be heretics.)