A theory I have no evidence whatsoever to support, but could possibly fit,
Perhaps Kemmler managed to pull off the Darkhallow. But perhaps it was diverted to a mostly deserted part of Russia so there wouldn't be the death toll needed to fuel his ascension. A failed Darkhallow could weaken Kemmler enough to make it possible to kill him off for good. Especially if whatever he did to keep himself from getting killed had to be turned off so he could fundamentally change himself to ascend.