I am not sure there was a "post" before the spring of 1863 when Harrison was appointed post commander. Of course, Fayetteville was occupied by Federal troops in December 1862 but I am not sure it had a "post commander" other than whoever was in charge of the troops.

William Shea in his Prairie Grove book talks about a lot of Federal forces stationed at Elk Horn and moving through Fayetteville. On page 265 Shea talks about the Federal forces staying around Prairie Grove much of December after Prairie Grove and Schofield was coming to take command and until then Blunt was in charge. It appears by Shea's book they spent most of December in and around Prairie Grove and the Van Buren raid. It appears Federal troop strenght gradually deminished until Harrison was left in charge of Fayetteville in the spring of 1863.

As I understand it, the answer to your question is there wasn't a Federal post at Fayetteville in December 1862.

Of course, I am sure there was an officer in charge of whatever troops were stationed at Fayetteville in December 1862 but I have yet to find a source of that information.

Quintin