Dagoth Ur was "slain" after the heart was destroyed. However, Nerevar bested him in physical combat before the heart was destroyed. Vivec could be killed before the heart was destroyed. Nerevar bested Hircine in his hunt. He brought down the false gods. If he isn't at least demi-god status, he's being cheated in the lore department...
Vivec was never canonly killed before the destruction of the heart, and even if he was, doesn't being away from the heart for too long weaken him? From what I've read of his/her challenge involving Hircine, it was only an aspect of Hircine, not Hircine himself. Is that true? If the Nerevarine is a demi-god, then there are also a few others to add to the list. Cyrus, the Battlespire recruit, and the CoC would rival the Nerevarine in the power struggle. I've never played either of the spin-off, but Cyrus was called something similar to "Hoonding" and it sounded important and I also read that he defeated a dragon and Dagon. The Battlespire recruit also defeated Dagon. The CoC did not defeat Dagon, but he/she did defeat Umaril and he successfully became Pelinal's successor, and therefore, he/she is also a Shezarrine. Along with that, he/she defeated Jyggalag and became Sheogorath. Whether he succeeded in fully mantling Sheogorath or not is still up in the air, but all four of these people have impressive resumes. Are they all equal to demi-gods?
By the way, what is it with Dagon? Bethesda sure seems to love him. Even Almalexia fought Dagon and Dagon was involved with the Imperial Simulacrum, too.