Jim thanks for being willing to go back and dig into the Hebrew and the text. Now that we know what the possibilities are we must go a step further and look at context and other words which you don't appear to be doing. What in those first few chapters of Genesis gives you the idea that "Adam" is not a single person but a plural humanity? That is the info you are not sharing with us.
I won't speculate as to why that is for I honestly don't know therefore there is no need on my part to label you as liberal or evolutionist. But I am curious about how you came to your conclusion.
I look forward to your response.
As to whether or not Adam is a 'literal' or 'real person', if the "first man" or "first Adam" was not a literal person, why would the "second man" or "last Adam" be seen as a 'literal" or 'real person', either?
The one is presented just as 'literal' as the other, in I Cor. 15:45-49. And Paul identifies us, as Christians, with both individuals in this passage.
45 And so it is written,
“The first man Adam became a living being.”[
a] The
last Adam became a life-giving spirit.
46 However, the spiritual is not first, but the natural, and afterward the spiritual. 47 The
first man was of the earth,
made of dust; the
second Man is the Lord[
b] from heaven. 48 As
was the man of dust, so also
are those
who are made of dust; and as
is the heavenly Man, so also
are those
who are heavenly. 49 And as we have borne the image of t
he man of dust, we shall also bear[
c] the image of
the heavenly Man. (NKJV)
As to the question you asked about the 'plural' usage, although I'm not
Jim1999, that may be found here, among other places.
2Male and female created he them; and blessed them, and called their name Adam, in the day when they were created. (Gen. 5:2 - KJV)
2 He created them male and female, and blessed them and called them Mankind in the day they were created. (Gen. 5:2 - NKJV)
Besides the KJV, the BIS, GEN, WBT, RV, ASV, PES, and DBY, among others, render this Hebrew word as "Adam" in the sense of a 'proper' noun.
By contrast, besides the NKJV, the MCB, YLT, NIV, LIV, NASB, RSV, and HCSB, among others, render this in the 'generic' sense as either "man" or "mankind" or something similar.
The footnote of the ESV (as well as several other versions) covers this; the AMP gets the best of both worlds, here

, by giving both usages; and the JFB commentary has this to say.
- Genesis 5:2 Hebrew adam (Footnote - ESV)
He created them male and female and blessed them and named them [both]
Adam [Man] at the time they were created. (Gen. 5:2 - AMP)
Ge 5:1-32. Genealogy of the Patriarchs.
1. book of the generations-(See Ge 11:4).
Adam-used here either as the name of the first man, or of the human race generally.
Point is, none of these foregoing posts are (or were) necessarily incorrect, but neither are most telling the whole story, either.
Hope that helps, a little bit, anyhow.
Ed