That's quite a conspiracy theory, denouncing one of the greatest heroes of Cyrodiilic/human history.
The artifacts are given only to those with the Nine's blessing, Umaril would have no use for said artifacts, and Umaril is no Daedroth. The whole part with the prophet could not have been faked, either. He delivered the blessing of Talos to the CoC, which allowed the CoC to destroy Umaril's soul(the view was nice), perhaps even suggesting that the prophet is an avatar of Talos, along with some of his speech that may sound familiar. It seems very unlikely to have been planned by a Daedra/Umaril, but if it had been, that must have been one stupid Daedra/demi-god, and Umaril may have had a massive ego, but he knows the artifacts are not good for him, which is why he tried to stop you at one point(Aurorans in the chapel where you obtain the Mace of the Crusader).
He didn't want to take them from you to use them. He wanted to take them from you so no one could use them against him.
He needed them secured and he knew he couldn't collect them himself. So, he had some pawn collect them. But his plan backfired.
Because yes, he was very stupid. Just like the knights of the nine, who thought hiding all the artifacts in the same damn province would keep them safe forever. Just like Jagar Tharn before them, who thought hiding all the pieces of the staff on the same damn continent was good enough. He should have cast them all into the sea. (I mean, he has the power to banish the emperor to any Oblivion plane of his choosing, but he keeps the staff on Nirn? derrrr). One common trend I've found in all TES games is that I'm constantly surrounded by idiots.
Which makes the world feel real, doesn't it?
EDIT:
I'll give you another reason why Umaril is stupid.
Akatosh just finished murdering Mehrunes Dagon's avatar. Akatosh is currently pissed off and on a violent rampage and keeping a close eye on Cyrodil. Could Umaril possibly pick a worse time to start desecrating temples of the nine and leaving obvious calling cards to draw negative attention to himself?
The only reason you would desecrate a temple and kill innocent priests is to piss people off. There's no strategic value to it other than shock value. If Umaril was really interested in anything but committing suicide, he'd be a lot more subtle, like Dagon was.
Umaril might as well have just used a fire spell to etch a big message in the grass, "Hey, Akatosh, please smite me!"