the empire signed the concordat due to the ebbing of their strength and power.
they chose to save their own hides, ie. halt/slow their ebbing, at the expense of their subjects/allies.
that choosing was cowardly and immoral.
their true moral option was to negotiate for fair terms and then fight for the behalf of their subjects/allies when the thalmor proved to be evil.
ie. you don't choose the path of immorality because if you don't you might be wiped out.
Let me tell you something
The Thalmor were pretty much torturing, raping, killing, and performing experiments on
any resident of Tamriel they came across, the Empire fought until finally they had their backs against the wall during the battle of the Imperial City, it was there that the Emperor signed the treaty because his citizens were undergoing excruciating horrors brought on by the war. Nobody liked the treaty, the Emperor hated it.
When Ulfric throws a tantrum and builds an army (named after himself, how vain) all he does is prologue the Thalmor occupation by weakening the Empire, the only force that could stand a chance against the Aldmeri Dominion. It says so in Thalmor dossiers at the embassy, they want the war to continue because it's good for them. Furthermore, Ulfric is a power hungry, corrupt politician. If you go to the peace council, all he does is kick and scream until he gets land while everyone else looks at him like a child throwing a tantrum in the super market.
Ulfric is also xenophobic, look at how Elves are treated in Windhelm. He doesn't care for them, he hates them, he promotes violence against them. Look at Riften, where Dark Elves are given a chance, there they are beneficial citizens who are treated equally. Let's not forget about the Markarth Incident where Ulfric ordered the executions of NORD men, women and children who he was supposed to be reclaiming the city for. He also tortured citizens to find out where the Forsworn were hiding.
Oh, and no, Torygg wasn't killed in fair combat. Talk to his wife, his wizard, everybody that was present. Hell, talk to him in Sovngarde and he'll tell you that Ulfric ripped him apart with a shout, it was not honorable in the slightest.