I was getting this as well. In any way, it seems to me that the "oaths" (if you can even call them that) for the Heralds are a completely different thing than what we've seen with the Radiants so far. It's not something you can work up to, more like a contract you sign.
And of course, you also have to mean it, and really mean it, which is why the Stormfather said that randomly trying words wouldn't work. I guess in Gavilar's case he was missing the point of what a Herald even is supposed to be, which is why it didn't work for him. So even if he was willing and was (in his opinion) ready, it didn't work. So close. And so far.