I have never stayed at Premier AUH, so this is just a guess, but... I think that you should be able to check into the hotel at any time. You just might have to pay for the prior / current hotel night, i.e. if you check in at 10:00, you will have to pay for the full hotel night that is just finishing up. In your case you would have to pay for 2 nights. You may also want to call/email the hotel directly to verify/clarify.

You may also want to post a query or at least review eaelier threads in the Etihad Guest forum. Quite a bit of stopover info available there.