So, it's deader than dead, right? No chance of Umbra ever making another appearance?

it was destroyed in the Novels, was it not?

while it does make it seem unlikely.. Umbra was technically a Daedric Artifact, and the Daedric Artifacts have been shown to be unpredictible in nature.. it would not be the first time a supposedly destroyed Daedric Relic somehow showed up again..

It looks to be in CV's nature to not learn a damn thing. Give it a while, and CV will conjure it back and make the mostly same mistake as last time.

I doubt it. It was destroyed in a more complete sense in the novels. Umbra is the main antagonist (in several forms.)