I'm not sure if there's anything formal, but it's definitely implied that Torygg's line has the Empire's support for continuing to be High Kings. That's why he inherited the throne from his father (who, it should be noted, Ulfric grudgingly notes was much better than his son).
Backing Elisif to succeed him is seen as being in part with this- she may or may not be a good candidate on her own, but she is the one who is most closely tied to Imperial interests. Even if their support is 100% sincere, it certainly works out for the Empire if she takes the crown...they'll have had a major part in putting her there, and they know she'll know it.
Okay, since you asked so nicely, I'll bite. First, I have to say that I am not a lore master like Matt, Phil, or Tom. I'm just a novice, so bear that in mind when you read this. This is going to be a long one, so brace yourselves . . .
The tradition of challenging the sitting High King to a duel is most definitely an ancient Nord custom. No one can really deny that. I see two issues at play, here: (1) Imperial culture vs. ancient Nord culture; and (2) Use of the Thu'um.
Imperial culture vs. ancient Nord culture
No one can deny that the custom of challenging the High King to a duel had not been practiced in a long time. If it were, why it be a shock to many citizens, including those who pledge allegiance to Ulfric? Very rarely--if at all--do we encounter a citizen who says very coldly, "Yeah, Ulfric challenged the High King according to our customs." On the contrary, citizens appeal to ancient Nord traditions and customs to legitimize Ulfric's actions. Why? I believe that Imperial culture has so seeped into the fabric of Skyrim and her citizens that ancient Nord anything is seen as unusual. This is easy to understand. The Empire has been the dominate force since the second era. The Empire brought to Skyrim prosperous trade, safety, and a new way of civilization. Apart from the Stormcloaks, there are no Nord armies--just Nords making up the backbone of the Imperial Legion. If the Dunmer were to attack Skyrim, the Legion would protect her, not Nord armies.
All this to say that Nord culture has "morphed" (for a lack of a better word) into a Skyrimized Imperial culture. For better. (we can argue all day about the Septim vs. Mede dynasties, but that's not the subject). Was Ulfric's actions, then, legal? No, not according to Imperial law. Skyrim has followed Imperial law, admittedly, with some autonomy, since the second era.
Here’s an example. During the colonial days of U.S. history, a person was punished for their crimes by being placed in the stocks in the public square. Now, that ancient custom seems strange, silly, and uncivilized 200 years later. However, here comes the traditionalist. He intercepts a convicted felon on the way to prison and places him in a home-made stocks. Hey, it's a return to tradition held by all Americans. While mildly amusing, you'd still have some Americans in favor of it (believe it or not). However, that wouldn't make it right. In fact, it would be illegal. One can argue for a return to ancient custom without breaking curent law.
Let's face it, Nords are pretty confused about their own customs. In Nords Arise!, the author claims that Talos blessed Ulfric with the Thu'um. We know that Akatosh granted the gift. What other misinterpreted customs do the Nords claim for support?
Interesting side note: nothing in-game indicates that anyone before Ulfric even thought about challenging Torygg--even the Jarls who despised Torygg.
Use of the Thu'um
Even if you think that Ulfric's duel was legal, using the Thu'um was amazingly unfair. First, the Thu'um had not been used in combat in over 4,000 years. Why would Torygg expect to face Ulfric's shout? How could he prepare for it? Second, even when the Thu'um was used in combat, only a select group of Nords even had this power. The very fact that Thu'um-users had their own unique title, Tongues, tells me that this group of people was small (compared to the rest of Skyrim) and unique. Granted, all fights have a measure of disproportionality. Someone will always be better. However, using the Thu'um against someone who would never have that power, is akin to David and Goliath, and Goliath (the nine-foot freak) won. It would only be fair if Torygg could shout, and even if Torygg was not as good at shouting as Ulfric, I would not call the fight unfair. Tullius and the Jarls aligned with the Empire appear to be shocked with Ulfric's shouting. You hear less about the duel itself and more about the morality of Ulfric's use of shouting. Tullius, at Helgen, says, "But a hero doesn't use a power like the Voice to murder his king and usurp his throne."
Last Thoughts
I like Ulfric as a character because of the controversy he engenders. However, I do think that he illegally and immorally performed the duel. I think he's power-hungry and somewhat self-serving (read: Nords Arise!). Nevertheless, I think Skyrim has a valid claim against the Imperial ratification of the White-Gold Concordat.
However, a united humanity is essential for mankind's survival. Human unity is a consistent theme through ES history from the Dawn Era War of the Ehlnofey through the First through Third Empires. Mortal men will not survive the Dominion scattered and fractured.
Skyrim was under the Empire's rule with a High King that answered to the Empire; Ulfric did not have the legal ground to kill High King Torygg, nor did Ulfric need it as in his mind, killing Torygg in the old way was the first step to freedom from Imperial rule.
Arguing morally will largely be futile but to have some fun, arguably. The Empire unequivocal win in matters of pragmatism but allowing its vassals to be tortured by a foreign power - and in turn making self defense illegal - does the Empire no credit.
I cannot fault the Stormcloaks on those grounds and had Torygg declared independence, a lot of bloodshed may have been avoided. Or perhaps Ulfric desired the throne for some time and knew Torygg stood no chance against his Thu'um, although Ulfric does not seem like someone who would use the voice for power grabbing.
Every single Nord has the ability to learn the Thuum. It is similar to how the Bosmer have the Wild Hunt, except the Bosmer can use that without training.
Ulfric was protected by law to challenge the High King. So really it was Torrygs fault for not being prepared to face someone probably twice his age.
ANY leader who accepts surrender under some kind of threat is no leader. Torygg didn't stand down just so he could live. He probably knew he stood no chance against a veteran like Ulfric, shout or no shout. That's just part of the burden of leadership, so choosing to fight over having an impression of a weak high king should not be one of them.
Well, in regards to the question, the answer is leaning more towards "beaten in fair combat". Ulfric didn't sneak into Torygg's bedroom in the middle of the night and "shout him to pieces", did he?
I think the answer to the question is "Tor Tor was beaten in combat". All's fair in love and war, right?
But a hero doesn't use a power like the Voice to murder his king and usurp his throne.
Tell me about it
- Forsworn
The, "Why doesn't the Dragonborn just kill everyone and become High King/Queen themselves?" bit has never really impressed me. At best, it raises some semi-interesting questions on the nature of "choice" in videogames and how we're really still restricted to a handful of pre-selected paths*. At worst, it comes across as a self-serving argument that seems designed to troll anyone who's actually attempting to engage the issue. The question at the top isn't, "Who should be High King?", it's "Was Ulfric's killing of Torygg legal/justifiable?"
The real answer to why the Dragonborn can't become High King/Queen is, "Because the developers didn't want you to."
EDIT: I should probably add the *. Basically, something tabletop RPGs like Dungeons & Dragons do well is put complete control in the players' hands; the only limits on what you can do are what the other people at the table will let you get away with.
But people always bring up Ulfric winning the duel and if we decide leadership by personal strength, then the Last Dragonborn is inarguably THE perfect candidate.
Is that really what's going on, though? The duel isn't an Official Leadership Test - Ulfric could challenge Torygg because Torygg was the Jarl of Haafingar, not because he was High King. (that is, as equals) I saw the ritualized duel as a cultural way of letting jarls who hated each other settle their fight personally, rather than having them pit their entire holds against each other. It's a way to resolve conflicts, in other words. It just happened to also allow a disgruntled jarl to bump off the current High King and force a leadership change...which then erupted into open war when the jarls lined up evenly on each side.
Ulfric had general contempt for Torygg, not just Torygg's leadership skills. The "If you can't protect yourself, how can you protect your kingdom" struck me as metaphorical, rather than a literal "might makes right" statement. These aren't Klingons or Orcs; they believe in strength, but they don't literally just follow the toughest person around. (Look at how many jarls are very old, very young or otherwise infirm. Idgrod's a sharp woman, but I doubt she'd win many swordfights)
If Ulfric, or the Nords, thought the physically toughest person automatically was the best leader, then Ulfric would have just done the same to every Imperial-aligned jarl that he did to Torygg - challenge them to a duel, then kill them and take their holds. He explicitly doesn't do that, because he wants to show that he also is a capable leader. ("He's shown them what he can do personally, now he seeks to prove the strength of his army." - a semi-exact quote from an NPC whose name I forget)
As is often the case, I think the "fact" that "Ulfric killed Torygg to show he's a better leader" is based more on what people have heard other, anti-Stormcloak players say online than on an actual analysis of the game.
These things tend to wander a lot, because they usually start with a specific question, then devolve into reflexive partisan bickering and people taking cheap shots at whatever aspect of the other side's position pops into their heads.
So, pretty much like a meeting of Congress. Ba-zing!