I agree that the actual events of the challenge and the duel are open to dispute and interpretation - after all, we weren't there and have to rely totally on what we hear about it, all of which is provided either by gossiping NPCs who also were not present or by NPCs who were there but have a vested interest in their own versions of the story. But I honestly don't see Ulfric's use of the Thu'um as a major problem with regard to the Way of the Voice. Had he still been at High Hrothgar, either studying to be a Greybeard or an accepted, acting member of their order, and gone to Solitude and used it in the duel, it would be different. But that wasn't the case.
I agree. Sybille's (or whatever her name is) explanation might be a bit fabricated, even though she was close to Torygg. Ulfric might be charismatic, but it is ridiculous to assume his view is unbiased. But concerning Ulfric's use of the Thu'um, he learned it while training to be a greybeard. Even if he abandoned the training, he should have been their long enough to know that the Thu'um is not a power to be exploited for personal gain. He had no reason to use it in the duel if he is truly a superior warrior.
I'm the Dovahkiin, and they helped me learn to do the same thing they taught Ulfric to do (altho being Dragonborn does give me a leg up in that respect), and then they let me go back out into the world to do as I see fit with it. And my use of the Thu'um is far more varied and powerful than anything Ulfric can manage. I did not have to pledge allegiance to their philosophy, I did not have to take a vow only to use it in situations that would meet with their approval. At most I get a little lecture warning me to be thoughtful about how and why I use it, and about the responsibility that comes with being free to make my own choices in that regard. I don't consider it unreasonable to believe that Ulfric got pretty much the same lecture, lol, and was cut loose just like I was.
The greybeards tell you that you are not bound by the same rules since you are the Dragonborn, and you were not required to take a vow. Ulfric, on the other hand, was training as a greybeard and recieved a much different lecture.