I like your theory, though there are some flaws;
And the art of skin-changing is passed down from generation to generation.
Sorry, I cannot conceed that this art was passed via generations, because the
race of
Men were simply unable to skin-change until
Beorn himself. There is no evidence to support this theory at all. This is a talent that only
Maia and
Elves had the ability to perform. Nowhere in the Histories of
Arda have we heard of any Man able to skin-change. Only Beorn, and his direct descendants have managed to successfully change form.
I find it highly impossible as I do not believe that
Tolkien would in some ways 'copy' Rudyard Kipling
Why is it so inconcievable that the seed of an idea may be put into the
authors mind? Surely Tolkien has drawn from so many sources both literary and mythological to come up with his final masterpiece. Everyone finds inspiration from both likely and unlikely sources, you and I could never prove or disprove this.
The concept that Beorn was of the line of
Elros and of the lineage of
Beren is certainly possible, however, as I have shown earlier;
Tolkien makes it clear in his letters that Beorn's lifespan was no greater than that of an ordinary Man.
From
the Silmarillion;
But to Elros, who chose to be a King of Men, still a great span of years was allotted, many times that of the Men of
Middle-earth; and all his line, the
kings and lords of the royal house, had long life even according to the measure of the
Númenóreans.
So the lifespan of the direct line of Elros was greater than that of ordinary man. So either, he is a direct descendant, blessed with both an ability to skin-change (how?) and longevity of years (No), or he is a distant, diluted descendant, with the normal lifespan of a man and a gene that has strangely missed generation upon generation, that originated in Beren, who never even had the ability to skin-change himself anyway? I find both of these scenarios extremely flawed, so I will stick to my own theory, unless of course you can convince me otherwise.