The 2 remaining Darkin will be Morgana and Sivir

Morgana?

Probably.

Sivir?

Hell no.

He was turned mortal. How would you pull out an immortal's heart?

Tell me, exactly, how would you go about ripping out Xerath's heart?

I'm not sure what you're trying to get at here.

I'm completely aware that was turned into a mortal.

I never implied that it was possible to rip out the heart of an Ascended.

That's because Ta'anari simply just hasn't been awoken yet.

Again, this is still assuming that Ta'anari is still a Darkin.

Which, he isn't.

Besides, the prophecy states that Sivir

It doesn't.

It says that Sivunas Alahair, The Bringer of Rains, will be the one.

Sivir may or may not be this "Bringer of Rains".

will either unite the world, or cause ultimate destruction, which to me, sounds like she will either conquer the Darkin, like Kayn does, or she will succumb to it.... like Kayn does.

While I will agree that this does seem to suggest that the weapon will turn out to be some kind of pseudo-Darkin weapon, you outright stating that Sivir will be the last Darkin still implies that she's going to lose this battle.

Which, she hasn't.

At least not yet.