So what's the key/stone to the throat of the world? Is there anything to point it out or any theories on it?

I heard something about time being the key/stone and that was shattered on the throat of the world by the Elder Scroll (Dragon) and the ancient nord heroes.

Well I don't know about that, it could be the notched pickaxe or the smashed word wall for all I know.

I don't think it's ever been stated. Theories range from the Eye of Magnus to Alduin himself to the Thu'um.

Though since the prophecy of the LDB mentions that the snow tower is sundered that means it was deactivated, so we may never know what the stone was.