Hmm. I like this idea but there are a couple of things that make me think it’s probably not true. The first is that presumably the Stormfather would be able to tell the difference between Odium and an Unmade. The second is Dalinar experiencing the divine, burning power. That would seem to be beyond the capabilities of an Unmade.
I wouldn’t say that I was disappointed with how Odium was portrayed but it did bother me a little that he was introduced so early in the series, meaning that Brandon has already played that card and obviously the mystery and menace that Odium poses has decreased going into the next book. Especially now that he seemingly has to avoid Dalinar.