A much belated update for anyone who might be interested in how this turned out.
Agent DID expect us to pay a commission; it turned up on the HUD form when the closing attorney in MS finally got everything sorted out and got a copy to us a day or two before closing. At that point, I advised him of the general situation as laid out in my posts here, he looked over the sales contract and other papers, and essentially said "Hmmm, you're right, she screwed up."
Keeping the lawyer handling probate in MS in the loop, he advised that since the sales contract was part of what he had to include in his presentations in court to get permission to clear title, sell the property, etc., that, as executor, I could make no offer of any extra payment as a "compromise" in the situation, but would have to start over in a fairly time (and money)-consuming process to rework things if I wanted to pay her anything. (Aside: He had a very low opinion of RE agents, in general.)
The closing attorney passed along this info, which resulted in the agent phone-harrassing both attorneys and me...I volunteered very little information to her and finally told her to only contact me through the probate lawyer.
The night before closing, the closing lawyer called me and asked if I wanted to just handle my part of the closing later in the day from the buyer's part, where the agent would be present (the buyer was not interested in delaying or giving up the deal). We did that, and avoided further unpleasantness. I don't know if he paid her anything on this deal, but the buyer later listed the renovated house with the same agent, so she wasn't completely left out.