Nietzsche does explicitly name a few people in Will to Power that he thinks rank among the greatest human beings that have ever lived, and he puts them in this category for traits very similar to those that he ascribes to an Übermensch:
Systematic falsification of history; so that it may provide the proof of moral valuation:
a. decline of a people and corruption;
b. rise of a people and virtue;
c. zenith of a people ("its culture") as consequence of moral evaluation.
Systematic falsification of great human beings, the great creators, the great epochs:
one desires that faith should be the distinguishing mark of the great: but slackness, skepticism, "immorality," the right to throw off a faith, belong to greatness (Caesar, also Homer, Aristophanes, Leonardo, Goethe). One always suppresses the main thing, their "freedom of will"—
—Will to Power, Book Two: 380 (transl. Kaufmann)
He praises Goethe in a number of his works for having been a "free spirit," something that Nietzsche himself expressed desire to be. And again, we recall that the Übermensch is one who embodies the free spirit par excellence.
In Twilight of the Idols, IX §49, Nietzsche writes with regard to Goethe's Dionysian character:
Goethe—…a grand attempt to overcome the eighteenth century through a return to nature, through a going-up to the naturalness of the Renaissance, a kind of self-overcoming on the part of that century…He did not sever himself from life, he placed himself within it…and took as much as possible upon himself, above himself, within himself. What he aspired to was totality; he strove against the separation of reason, sensibility, emotion, will…; he disciplined himself to a whole, he created himself… Goethe conceived of a strong, highly cultured human being who, keeping himself in check and having reverence for himself, dares to allow himself the whole compass and wealth of naturalness, who is strong enough for this freedom; a man of tolerance, not out of weakness but out of strength, because he knows how to employ to his advantage what would destroy an average nature; a man to whom nothing is forbidden, except it be weakness, whether that weakness be called vice or virtue… A spirit thus emancipated stands in the middle of the universe with a joyful and trusting fatalism, in the faith that only what is separate and individual may be rejected, that in the totality everything is redeemed and affirmed—he no longer denies… But such a faith is the highest of all possible faiths: I have baptised it with the name Dionysus.
It's certainly poetic, but in addition to Goethe, Nietzsche also offers up great admiration (most pronounced in his earlier works) for Schopenhauer, a man who was able to reconcile action and contemplation, and voluntarily chose to give up both comfort and happiness in life, courageously confronting the suffering of the human condition in a positive and affirming way. Schopenhauer seemed to Nietzsche to have transformed life's many hardships into a cheerful disposition, a characteristically Übermenschlich activity. He is also quite taken with Rousseau, whom he sees as a revolutionary.
Thus, I remain skeptical of any pursuit to identify the true Übermensch, as per Nietzsche. He wasn't particularly bashful, and it's unlikely that he would have avoided naming someone directly if he thought they really merited the title. It's quite important throughout Nietzsche's work that no one has ever truly achieved the status of Übermensch. It's why he himself found the concept difficult to describe at times. Note that he describes the Übermensch as "belonging to the future": he has never fully existed, neither in the being of Goethe nor of anyone else.
In my opinion, few academics have taken up the question of who is/was the "true" Übermensch because such a question is irrelevant and might, in fact, be harmful, at least to the extent that it detracts from the fundamental point that Nietzsche is trying to make.
As far as other scholarly references, the best one I know of dealing with extra-literary questions like this one is Kaufmann's own Nietzsche: philosopher, psychologist, antichrist. He devotes several sections to a discussion on the Übermensch concept, and mentions Goethe (and the other contenders) specifically.
You might also find this article interesting (if you have access to scholarly journals through a service like Project Muse). I remember reading it myself a while back, and found it quite insightful. Re-reading the abstract, it looks like it might be pertinent to some of your questions.