Creation really is just an experiment
If something that humans created (The Way) have thrown that big a wrench in God's plan kind of makes him not so omnipotent after all. Mirsky isn't Mirsky at all in Eternity
He's simply a copy - a partial, if you will - sent by the Powers That Be in A Form You Are Comfortable With. The Talsit are humans from the far future
Hey, if the stone can end up getting whipped back into its own alternate past, then The Way can open up into what could be their alternate future. As mentioned in the book, the Talsit are actually representatives for "a much older race". The older race keep themselves secret from the humans because they're humans, too. Frants are all french.
They come from Frants! Olmy, by Eternity, is suicidal.
Hence why he stupidly takes the Jart on. Then he goes off to be alone in the woods and wait for the end.