With Ulfric and the Jarls that support him, do you really think that Torygg would have been allowed to remain High King? That's what I meant when I said he'd have to step down if he refused the duel. Torygg would not have survived the Moot.
Eh, Ulfric did what he did to get more support for his side. Even with the support he has, he doesn't have a majority of the Jarls with him. With less support, he'd have an even smaller minority. I don't think it is likely Torygg would have lost the moot -- and to be clear, it is his position that might not have survived, not his life (hopefully I'm being pedantic by stating this). He would, however, have lost a lot of face and lost self-respect due to how the Nords define honor, which is why he didn't say "no", and in fact it is why psychologically it would be extremely difficult for a Nord to say "no" to such a duel.
The Empire shouldn't set Skyrim's laws, the High King should. He accepted the duel and faced the consequences. But even if it was not legal, Roggvir didn't need to be executed. The court wizard, the guards that were there in the court, and pretty much everyone else along the way from Blue Palace to the gate that Roggvir opened could have stopped him...but they didn't. They are as guilty as Roggvir would have been, if the duel was truly illegal.
Skyrim is part of the Empire, and in fact Talos founded the third Empire. You seem to be ignoring that and acting like Skyrim is some sort of independent country. It's like saying a district/county/state in a modern country should ignore the laws of their home nation.
As for not stopping Ulfric earlier, that gets into the complicated cultural and other issues that make it all very interesting. Part of it is because the High King said to allow the duel, part of it is likely the shock of him getting killed so quickly and the shock of a duel actually happening, part of it is likely some confusion given the tradition, and part of it is because it seems Ulfric got out of there real quick. It seems by the time things got to Rogvir, that confusion and shock was over, as the game makes it pretty clear it was understood that his legal duty was to stop Ulfric. Rogvir knew that was the written law, but decided tradition overruled it -- that's his defense for himself, anyhow.
If you allow that a tradition is not a law, as you should, then that makes what Rogvir did
treason. Which most countries today and even historically have always punished with death. So I don't see how any other punishment would make sense.