On where Loki's billions of souls come from, they don't even have to come from the current world. Odin's currently senile because the previous world thought magic was only believed in by idiots. So consider the effect of thousands of worlds of people believing that Loki can't tell the truth, even if some of them exist for only a few years.
Loki very clearly was talking about those currently alive though, not any lingering effects from past worlds.
I'd just like to point out that saying that something unsupported is the case unless someone else can prove that it is not is an utter failure of logic. - Kish