About your claim #3 - You are overlooking that steps 4-9 deal with the "deity" of Christ. I am focusing on the sense of "deity" or "divinity" which implies that the thing is a god. Compare: human. In the primary or basic sense of that, whatever "is human" just is a certain human being, e.g. Steve.
Fine. Let's play along with Tuggy's own example.
i) Is mankind one or many? You can say mankind is only one with respect to the fact that there's only one human genus. That's why it's called mankind or humankind.
ii) But, of course, there can be many representatives of that kind or genus. Indeed, there seems to be no intrinsic upper maxima to the possible number of human representatives.
iii) By analogy, consider God-kind or God as a genus. At that level, there's only one. God is a class by itself (or himself).
Yet there can be more than one representative of God-kind. Indeed, there are three. Unlike humans, that does have an intrinsic upper maxima.
iv) There are differences, of course. Humans are finite, concrete exemplifications of God's idea for each human individual.
By contrast, the members of the Trinity are more like abstract mirror symmetries. Each one reflects the other two. Contains the other two.
v) However, these refinements are irrelevant to the larger point that there's more than one way to count certain things. In some cases it's possible to count the same thing as one or more than one.