So you're implying Ulfric should've just sat there and been killed by the empire for trying to follow the nordic custom. Which also appeared to be a PERFECTLY LEGAL way to challenge leadership in High Rock. The empire never accused their duels of being against imperial law. They've never considered the same in their own legion forces of being against imperial law.
No, I'm implying that Ulfric should have either sat there and let his would-be subjects, the Nords of Solitude (who are no less equal than the Nords of Windhelm), determine what to do with him, or return to Windhelm and wait for a moot. From what I understand, he started the rebellion before Tullius and the Imperial Legion showed up. Chances are, if he didn't rebel, and if he did get to attend the moot without any Imperial interference, Elisef probably would have been picked as High Queen because according to Nordic customs, she's got the strongest claim. She's also alienated far fewer people in Skyrim than Ulfric had at that point. As far as I know, he had every intention to ensure he became High King, and part of that involved rebelling and ensuring that when the moot was called, the Jarls would be loyal to him. And with the Thalmor pulling his strings, there was no chance it would end peacefully.
It's hard to come in for a moot when the people involved keep trying to kill you.
It's also hard to get elected High King when the people involved (and by that, I mean the Jarls) think someone else has more of a claim by your own customs. Torygg won the last moot over Ulfric simply for being the last High King's son and that was back when Dengeir was in charge of Falkreach. Falk Firebeard says that Elisef's claim to High Queen is itself Nordic custom.
I'd love a source on this. Far as I saw he voices his opinion on the matter and then waited to see if anything came of it. Nothing ever came so he went the challenge route.
The source for the moot is dialogue from Sybille Stentor:
Even after Istlod died, the moot voted to make Torygg High King of Skyrim.If Ulfric had respect for the results, he wouldn't have challenged Torygg. Don't forget that Torygg in fact respected Ulfric and might have sided with him. But Ulfric wanted to be High King, and we both know he felt Torygg was too weak (in terms of a warrior) to be High King of Skyrim. The challenge was a lose-lose for Torygg and Ulfric knew it. If Torygg declined, he would be seen as a dishonorable coward (turning down a duel is generally viewed negatively, no matter how unfair it would have been). If he accepted and defended his throne and honor, he wouldn't stand a chance.
And they would have done it anyway. Are you saying the empire would stop them?
No, I'm implying that they probably would have sought a civil war in the Empire even if they didn't have a rallying figure like Ulfric. We know that if the Imperials win any time soon, it'll weaken their overall position in Skyrim.