Wondering is there a way to make it permanent if you hand Auriel's Bow to Harkon?

You can't hand the bow to Harkon, so no.
You can't make it permanent, but the arrows are so easy to get I don't see that as a problem.
I actually really like how they did it.

I don't think so. That would be really drastic, but I don't know for sure. Since you can craft arrows now, you should just make a giant supply if anything. I have about 5k Bloodcursed Elven Arrows and 8k Sunhallowed Arrows to use regularly (exploding arrows are amazing).

You can, if your on the Dawnguard side, but i think you still end up fighting Harkon, and he dies and you take it back xD

Nope, I believe you have to kill him with it

Vamp side too. Either way you can give it to him, and either way he tries to kill you.

Killed him while in Vamp lord form. No bow necessary.

Yes but you should be able to choose to side with harkon.

And Ritualistically sacrifice Serana to the Dark Lord, King of [censored], Lord of Dominition. Molag Bal!

One Question, would afore mentioned sacrifice be commit with a normal knife? Or would you have to use a certain appendage to murder her?

Just out of curiosity, would killing Serana blot the sun permanately? Because if not, Harkon's way is just stupid since Serana donates her blood to my arrows.