Wait what? When was Wit speakng Dawnchant? I know he used that word when asking Dalinar about Adonalsium, but I don't see how the "gibberish" he then proceeded to use has anything to do with the Dawnchant. Sure, they might, but to me it seems more likely that he was indeed speaking gibberish just to take Dalinar's attention away from the one real word he did use, i. e. Adonalsium.
And yes, it is interesting to note that Dalinar's Highstorm language was indeed described as gibberish. From what I understand about Brandon as a writer, it could indeed be the type of subtle easter-egg he would use, but it is still mightily weak grounds for assuming the drunkard was indeed speaking in Dawnchant. And after a careful reread of the prologue, he is nowhere referred to as drooling.
It would be surprising, yes, but that's assuming Szeth's Shardblade is an Honorblade in the first place. I am not entirely convinced it is.
Also, the only thing Brandon has said is that all Heralds were either seen or mentioned in tWoK. As far as I know, he did not say to what extent, so the mention and viewing Jezrien got in the Prelude might be all he ever got in the book. In my opinion, there is just WAY to little to go on here. Of course, he might still be Jezrien, but at this point, wehave no firm evidence, only maybe shadows of possible would-be hints and red-herrings.