I've been away so this might be too late to be seen, but ...
My basic point which you either miss or don't agree with is that poetic license allows even a noted professor of speech to make grammatical mistakes IN A LYRIC (not dialogue) as long as the line makes the effect it's supposed to make. Grammar just doesn't matter.
Sorry, but your version of the line is not as good as Lerner's. "I'd be equally as willing" just sounds better than "I would equally be willing" and "than" makes a better effect than "as". I don't give a rat's ass that yours is more grammatically correct. My guess is that Lerner probably thought of your version and other more correct versions, but just knew his was better.
For lyrics, effect trumps correctness, even for professors of speech.