There's a difference between the Greek word and the functional usage of icon in iconography.
What do you mean by "the functional usage of an icon"? Not all traditions venerate icons in the same fashion (in the Coptic Orthodox Church, and apparently in at least some of the Syrian churches, we 'kiss' them by touching them and bringing our fingers to our lips, while the EO kiss the icon itself with their lips, etc.), make use of them to the same degree, etc.
An icon is a representation that is necessarily inferior, lacking in some substantial way.
I don't agree. I'd say that an icon is different (we don't interact with them in all the same ways as we do the scriptures, depending), but not inferior. Besides, what do you then make of illustrated gospel manuscripts, like the Garima Gospels in Ethiopia (perhaps the earliest example of this phenomenon in the entire world), or the many similar examples we can find in the Armenian and Syrian traditions? These really call into question the distinction between the image and the written word.
Christ as the image of God does not express the fullness of Christ as the substance of God, though because Christ is both image and substance both can be said of Him.
So why would you separate the two? On the contrary, I would say that Christ can only truly be called the image of the invisible God because He makes the invisible God physically real before us, which entails or presupposes the
homoousian reality of the Holy Trinity, such that when we see Him, we have seen God. In other words, saying that He is the image of the invisible God is not saying that God the Father is however tall Jesus is, or has the same eye color, or anything else about physical characteristics that the word "image" may conjure up in our minds (so, in this strict sense I agree with you that there is a difference between
the word icon and what this all means theologically). Rather, He is the image of the invisible God
because He and the Father are One.
I don't think icons 'work', theologically or practically, without understanding this beforehand. Otherwise, yes, it is just a picture and is thereby necessarily inferior.
An icon does not have that substance, it is simply an image.
If we're talking about the icon as a material object (i.e., the mounted or framed painting), then sure. But note that the same would be true of the Bible: it's 'just' a book, of whatever dimensions it has, of whatever length it has depending on the typesetting or footnotes, etc. This way of approaching things is not illuminating, whether we are talking about the Bible, icons, or anything else. Something having whatever physical properties it has is not the point in any case.
The Scriptures don't merely paint a picture of Christ, they provide us with a means for relationship and delving into the inner experience of God.
As icons do as well.
While they do not have the substance of God, they are more than a simple image.
And neither are icons just simple images.