Like say, if Dagon ore the heart out of a Shazarine, and someone who saw it got maimed? Would this put Mehrunes in the same place as Lorkhan?

Okay first things first: Mantling is to become something like (not going deeper into that now) Aedra, eight gift-limbs to Nirn plus one Heartbone. CHIM (and Amaranth) is different, more like merging two sides of duality and thus becoming Godhead. It's related to duality of Universe: Anu and Padomay, ones who are not Et'ada but above them. Ones who actually are one. Enantiomorph. Amaranth is step where one becomes new Godhead dreaming his own dream.

So that dealt with.

Gods don't mantle, they are gods already. Lunar God is only known case to want something more (=CHIM and Amaranth) and he had to do it thru his children and Nirn, so while mortals tries to mantle their above, Lunar God as a God had to go down to mortals. One could even say that Lunar God can't become Amaranth and be free from this Dream, because Nu-Mantia (=freedom) is achievable only by progeny of Mortals and based on that even our current sentinent habitats of Tamriel can't achieve Amaranth. Thus Et'ada and Mortals are stuck in this dream, but good parents don't complain they just enjoy that their children eventually gets there into Nu-Mantia.

About mantling there is more interesting question: Can mortal mantlers such as Lorkhan rip heart of Auriel??? Why not. After all would be know if Emperor Zero would have killed Hjalti? Or Nerevar killed Vehk? No we would not, because they would just switch their places. Things naturally would have changed quite a deal, Elves would be telling stories of their Missing God Auriel while Lorkhan and his foul folk transcedenced to Aether. We would have Alinori Selectives and and such. But then things really would have not changed at all, same petty rivalries and stuff would keep on going only with switched roles.

Or.

Most people on Lore Forum would scold Nerevar for being such bastard to kill Vehk (and we would have Vehkine, which isn't nearly as cool). In Fould Murder we would have guy with mohawk sticked in Trueflame by guy with mohawk. [yawn]just woah[/yawn].

Or.

As for Hjalti and Emperor Zero... I really don't think nothing would have changed, names maybe. But then Emp. Zero gets name Tiber Septim and so on.

So yeah. As for Dagon and Shezarr. Well we don't know much about Shezarrs, main thing about them is that they/he popped up and dissapeared all over Tamriel during Nedic rebellion. I don't see why Dagon couldn't kill Shezarr already, but there isn't anything cool mythic about it. Dagon got just good laugh mostly. And Shezarr appeared somewhere else again. Shezarr after all was all about appearing to help Nedic people in need just to disappear again until when Aless eventually won and then Shezarr wasn't needed anymore. Not much to do with another Lunar God mantlee named Lorkhan, who based on Altmeri myth got killed just once and who's heart was shot into ocean. They are two different cultural Heroes. Like Nerevar and Emperor Zero and all those.

I've probably misspelled few things, no matter, no time to check.