Yeah, go with the earthquake excuse. ;-) The name is very often misspelled on old maps and correspondence. Even Brig Gen Stand Watie, who got his colonel's commission from McCulloch, misspelled the name in a June 29, 1865 letter written from:

Executive Office for the Cherokee Nation
Ft. McCullough, C.N. [Choctaw Nation]

Nail's Toll Bridge on the Butterfield Mail Route was adjacent to the fort so Price's men would have passed right by it.

There were probably a number of Cherokee refugees camped at what remained of the fort at the time.