Is the lvl 80 spell Charge available after doing the mage arena mini quest? Do you get it after you cast 100 god spells inside the arena, or do you have to cast the spell Charge 100 times aswell?

You have to be wearing the God cape.

Okay thank you,

 

Another one quickly, I know that you can have all three staves, but what about the capes, can you have all three of them at the same time?

No, only one cape at a time.