TMac,
By your definition of "Dean of" which I think is correct, and based on being the first american born gca, only Emmet would seem to be in contention, and Whitten lists Garden City as 1899 as opposed to Frankford's 1898 according to Phil.
If we add in my conjecture that Tillie would put accepting paid fees as a gca, and Whitten's statement that Emmet didn't accept fees until later in his career, then Tillie could stake that claim on either basis, no?
Did Emmet or other americans design courses that we know of before that? And, not that it matters, but if Tillie wasn't the first American to accept a fee to design a course, who would you say it was? You mention O'Neill as designing "around that time" but Whitten lists his courses as way later, but allows that he dabbled earlier. And, by your strict "dean of" definition, O'Neill (or others) would have to be proven to have designed a course in 1897 or earlier, no?
Of course, you might argue, and I might agree that Tillie was narrowing the definition quite a bit to suit his desire to make the claim (whether for business promotion or personal ego, its quite a distinction) American Born, takes a fee (or perhaps is just invited by outsiders in his first work) continues to take fees on regular basis, etc. I think there is probably enough truth to it to allow him to make that claim, no?