So, I haven't heard back from them in 5 days (I replied on Sunday evening). Does this mean it has gone to arbitration? Do I just need to wait it out
You seem to be under the impression that there is some semblance of honesty and independence with the IAS. Just wait for a response. Don't hold your breach and if it is unsuccessful (highly likely), the decision is not binding on you and you DO NOT pay. You just move on to the next phase.