Can you absorb more than your max magicka? For example if your magicka is currently at 200 out of 200 and you absorb an incoming spell with 100 cost will your magicka now be 300 or will it still be 200?

It does not absorb more than your total magicka.

No, your magicka pool limit will stay the same.

It can keep a character fighting a Dragon or Mages at almost full Magicka, but it does not overcharge a character.