1. We define "Chosen" as a template that can be added to a person that carries some ability changes and special powers. The particulars of each Chosen template vary a lot. Fzoul is the Chosen of Bane, and the template that he's got seems a lot different from the Chosen of Mystra template. I'd assume that some gods decide to create Chosen, and others don't, and that the powers of any particular Chosen are sort of dependent on the whims and abilities of the god in question.
2. Is everything Ed says canon? No. Canon is the assemblage of information in the current edition of the rulebooks. Beyond that is a much larger sphere of "we thinks" and "when we get tos" in which Ed's never-ending font of creative energy is quite prominent. For example, if Ed writes a couple of thousand words on Rethmar in the course of a bunch of posts to a message board, it's as good as anything until something else gets into print. In a perfect world, we'd know all about Ed's previous speculation on the topic and make sure it was part and parcel of any other designer's work on that city. Sometimes, it doesn't work out that way. Ed doesn't monitor all RPGs, novels, and PC games being worked on, and we don't keep track of everything he says. Heck, I don't *want* to. I don't want Ed to have to be afraid to speak from the heart about anything he cares to talk about, and I don't want to have to approve what he says before it goes in the public eye. Seems like that would make us both miserable.
In any event, I'm not sure that the whole concept of canon is worth the fuss we seem to invest in it. If something isn't in print, I don't treat it as canon. Even then, I'm willing to "evolve" canon when the situation calls for it (for instance, an edition shift in the D&D game). I know that some fans don't like that, but that's the way the business works.