While these are all neat ideas, about Adams children mixing with others, or others existing before Adam, I find them inconsistent with the scriptures, for instance, 2 Nephi says:
19 And after Adam and Eve had partaken of the forbidden fruit they were driven out of the garden of Eden, to till the earth.
20 And they have brought forth children; yea, even the family of all the earth.
Also in Moses:
7 And I, the Lord God, formed man from the dust of the ground, and breathed into his nostrils the breath of life; and man became a living dsoul, the first flesh upon the earth, the first man also; nevertheless, all things were before created; but spiritually were they created and made according to my word.
If anyone would like to read further, President Smith wrote a book entitled "Man, His Origin and Destiny", which deals with all these topics, he is very clear that Adam and Eve are the first parents of any and everyone who ever has or will live on this earth.