How are commission rates split?

There is no "standard" in the industry. In general, commissions are completely negotiable and depends on the specific agent or brokerage. Often the seller wants to minimize how much commission is offered out to one of the agents or both, though keep in mind that the commission is really an incentive for each agent to perform. The less you offer out, sometimes you can save some, though the less incentive there will be. The more incentive there is often will mean more service, care, diligence, and better results. This isnt always the case but something to consider.

I am not certain that all states are the same, but in PA, the commissions are negotiated and are specifically stated in the Listing Contract. There is no "standard", these are always negotiable.

Usually depends on the policies utilized by both firms. Nobody actually determines the commissions received from a transaction.