I'm inclined to agree with you on #1. I thought of a similar idea, actually. Merging slightly with yours (adding the Heralds/Dawncities, which I hadn't considered): Each Herald trained the Dawncities (and surrounding Kingdoms) in one particular area of expertise. This is based on Azir; I suspect that they were kind of the "bureaucrats" of the shadowdays, managing all the organization of Roshar, leading modern Azir into their ridiculous obsession with forms. Based on that theory, Alethkar would have been the warriors of Roshar. It would be weird for one country to have one specialty, but I feel like there should be some explanation of the form-obsession.
As for your other ideas regarding the Honorblade keys, that does seem like a good explanation for why only one Oathgate worked, which bothered me a little.