I have been researching the Bible for over ten years, plus a spiritual education, and I note for my part that you are well prepared! 
I would add that the tetramorphs are portrayed differently, not only in pictures, but also in the Bible itself. For example, in the book of Revelation there are four creatures near the throne of God, one with the face of an eagle, another with the face of a lion, etc. etc. These are the same creatures, but described differently, indicating that their appearance is described symbolically to us, not literally. The reason for this is that it is impossible to reliably describe a spiritual being in physical terms.
The connection of the four animals to the four evangelists is strained and unreliable. A more credible version is that each of the faces symbolizes the creature's responsibility (or authority) over the human family, the bird family, the wild animal family, and the agricultural family. But it is also a version.

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