When dovahkiin fought Alduin in Sovngarde what if instead he absorbed Alduins soul. Because when you absorb a dragon soul they say you get their knowledge and power. so if he absorbed Alduin in sovngarde could he have mantled him?

I don't think he (or she) absorbed anything. It looks like Alduin was kicked into some other dimension.. Or maybe "re-combined" with the "oversoul" of the Time Dragon (ahem just a guess). I think he was placed somewhere for a rainy day. There's still a need for a world eater probably.

No. Mantling means that he must act like Alduin until Alduin acts like him.