Yes to your question...but there are ways to smooth out the edges

Sounds goofy to me - either party can rescind/withdraw an offer or counter offer anytime prior to the other side accepting and returning the signed around PSA fully signed/initialed. I bet that's true in most states but I'm not sure. What counts as a "legal" withdraw or rescission can very from state to state. In WA state one can withdraw from an offer or counter offer verbally with a phone call - it's best to follow up with paperwork but when timing is critical a phone call will do it.

In California the answer is "yes" if the counter offer had not been accepted and returned to the seller but it would be necessary to withdraw the counter offer before accepting the new offer. The CO could be returned prior to the deadline and the contract would be made.

Much the same as Dan Pittsenbarger has set forth forth. 

It would also be possible to rescind the counter offer, evaluate the 2nd offer and make a counter offer back to one or both buyers.

I won't without first withdrawing the counter offer. You could find yourself with 2 contracts to sell one home.

The counter offer would need signed acceptance by the buyer and acknowledgement of delivery to / by the seller. Up until that point it can be withdrawn. AZ, YMMV.

yes, any counter can be withdrawn before it is signed.

YES in CA as that often happens when a Seller Multiple Counter Offer is issued. The deal is open on the table to all until ratified. You may ruffle feathers, but it's legal and ethical.

accept or entertain?

In my area they can counter, then another offer can come in & it's a dual now. Nothing much Buyer #1 can do about it.

Yes, in Florida you can. But if you accept that offer before it expires you might have an issue. 

I do not know Georgia Laws regarding real estate or otherwise. 

In California that would be a yes. 

Seller can accept any offer they want

yes

Best to email buyer agent about multiple offer and withdraw the counter before they respond

follow up with text or phone chat

The deadline is for the buyer and meaningless really, people will do what they do. The lessone for a buyer is do not wait and make your best offer first. Avoid some other buyer getting their offer in