Some good points smidge.
I always assumed the second quote was merely Dalinar overstating the Kings effectiveness in order to bolster his confidence, but I certainly could be wrong. Even his own mother admits he sucks as a king, so I have a hard time seeing him as the kingdom's savior.
Now for the first quote, we know the Dalinar that existed when the vengeance pact was made was not the studious, history reading kind of high prince...more the drunk in his cups when not fighting kind of high prince (like at the treaty signing celebration). I think the danger of the kingdom splitting seems to have increased over time as the high princes have been allowed to operate largely on their own on the shattered plains and that is what has Dalinar so concerned. Or in other words, his concerns feel more recent to me. I think the vengeance pact was not a desperate bid to keep the kingdom unified, but the way I read it, was a natural response of the war-like alethi people.
Regarding your last point, I don't know if I'm convinced. Szeth didn't say he had committed one of the worst slaughters of his life, but that he had committed one of the most heinous slaughters in all of Roshar, so the fact that it was Szeth's worst slaughter do date doesn't seem to matter. He appears to be measuring on a scale that is bigger than the scope of his own actions (as in All of Roshar). Furthermore, Roshar seems to be a place of war and intrigue, especially among the Alethi. They are the type of people that will send hundreds of bridgemen to slaughter without a second thought. With that in mind I find it hard to believe that the killing of a king and a few guards was one of the most heinous slaughters in all of Roshar.
Of course, maybe Szeth was referring to the fact that nobody can slaughter with as much ease as himself. It really is like shooting fish in a barrel for him, so maybe that is what makes it so heinous.