How else can you explain Ur's corruption and subsequent gaining of power? Unless someone explains to me where Ur got all that magickal experience that allowed him to get power from the Heart through some ritual he literally made up, then corruption makes the most sense.
I'm not saying that Lorkhan gave him all those ideas and whatnot; that semi doesn't make sense. But like I said, IN THE END it favored Lorkhan EXTREMELY, and he was one of the last few notable figures that interacted with Voryn prior to his demi-apotheosis, indirectly or otherwise.
Even if he was corrupted by the heart, that doesn't mean he was manipulated. Besides, if lorkhan was manipulating Dagoth, wouldn't it have been better to simply make him believe that striking the heart once with sunder, and then a lot with keening would produce a good result? Also Dagoth Ur seemed pretty adament that he did not like the nerevarine doing just that.
I see no reason to believe Dagoth was manipulated by Lorkhan, especially sense all he did was plan to imitate Kargenac. A coincidence is sometimes a coincidence.