First, wisdom is temporally subjective, and based almost entirely on perspective. In hindsight, Ner'zul was a fool. In Hindsight, Valen was too, for thinking Kil'Jaedan might see what he saw in Sargeras. Still, why don't we totally ignore the fact that we are told, matter of factly, that Ner'Zul was the wisest and most powerful of the pre-corruption Shamen. It of course has no bearing in anything, now does it?
The least foolish of a band of fools is still a fool. So no, it doesn't.
In the end, the Orcs were outclasses and basically doomed by a force eons above and beyond them. No power structure would have saved them simply due to the discrepancy in power between they and the Legion.
As to Velen, he wasn't a fool so much as he was hopeful - his power, however, was limited in comparison to that of Ner'zuhl, so his accountability is less.
Second, there are at least 2 other situations, directly from WoW, which show that your ideal is flawed, particularly in the Warcraft world. The Tribunal and the fall of the Eredar, and the corruption of the Council of Nobels leading to the riots in Stormwind and the formation of the Defias Brotherhood. Oh, then theres Thermaplug's manipulation of the Gnomish assembly. And the whole Klaxxi system has clearly worked out so well for them.
No, when i think about it, the Warchief system is alot more stable as it is without having to adopt any of this psudo-democratic, double tier checks and ballance nonsense.
In the case of the Eredar, it was them vs. the single most powerful evil entity known to exist by an unerring margin. The fact that he was able to convince 2/3, however, means that he was able to convince 2/3, and that last third gave the Burning Legion one of their single greatest enemies.
If all there was was Kil'jaden or Archimonde, we'd not have the Draenei. Because of the council system, some of the Eredar directly escaped corruption. So, this example actually proves the notion, rather than refutes it.
The Council of Nobles, meanwhile, also has little bearing on the argument - even the King himself was directly manipulated by Onyxia, and moreso to an even greater, nay, cartoonish extent. Here again the scope of the threat goes beyond political structure, and having all the power in the hands of one man would have made no difference. Initially, Varian actually TRIED to work things out between the nobles and Masons, so really, this is an argument FOR division of powers, as once again, having ALL the power in one hand would have prevented ANY sort of countermeasure no matter how effective or ineffective.
Thermaplug isn't even an argument against a council government - it was the King he directly convinced to microwave Gnomeregan. The Assembly is actually what prevented him from becoming king in the first place doing Light knows what sort of unspeakable things.
As to the Klaxxi, their absence and non-existence wouldn't help anything. They're still the best weapon against the Empress, regardless of the fact that they're reliant upon outsiders. This disaster is only DUE to Outsiders anyways, so it's only fair.
It's not, "they haven't completely solved the problem, therefor they're useless," it's, "if you think it's bad WITH them, how much worse would it be without them?"
Meanwhile, the absolute powers of the Orcish chieftains are what directly resulted in the second time corruption of the Warsong Clan, the alienation of most of the Horde of Kalimdor, the invasion of Gilneas and their resulting rejoining of the Alliance, and every other boneheaded thing you can assign to Garrosh. If Baine and Vol'jin were given an equal say amongst the Horde, it would be a far more efficient, unified, and sustainable entity. Now it's barreling towards civil war and self destruction, all because of ONE Orc and his insatiable war fetish.
The idea of the WarChief in and of itself is an archaic, barbaric, and foolish tradition that should have died out with drinking demon blood and working for the Legion.