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Diogeneslamp: "This Mikkelsen guy seems to have made a mistake in saying Hertz sources it to Elkins, because I find absolutely no reference to Elkins in the entire book."

According to Rich Buhler, who says the Lincoln quote is genuine, Hertz's book has two volumes, the alleged quote to Elkins is on page 954 of the second volume.
See my link in my post #76 above.
You might be looking at just the first volume.

Key point, the reference to Hertz's book doesn't only come from Mikkelson, but also, according to Buhler, it's confirmed by UC Davis researcher Rick Crawford.

Also, even if you find the alleged quote in Hertz's second volume (from 1931), where, exactly did Hertz get it?
The suggestion is that it came originally from Lincoln's alcoholic law partner, William Herndon, but there's no confirmation of that and so there's no real source for the quote before Hertz's book, and we now are not even certain of that!

Diogeneslamp: "One point though that nobody seems to have addressed; If this statement comes from a letter sent to Col. William F. Elkins, then wouldn't it be in Elkins papers, rather than Lincoln's?"

Sure, maybe, but nobody I've read has claimed that's where the quote came from.
Instead, they say it came from:

1951 Archer H. Shaw book which said it came from
1931 Emanuel Hertz'book two-volume Abraham Lincoln: A New Portrait, which may, or may not have taken it from
1889 William Herndon's three-volume biography of Lincoln.
Regardless, the alleged Lincoln quote was denounced as a forgery by Lincoln's secretary John Nicolay, according to Mikkelson.

And that is all I can find on it, so far...