I understand the distinction relating to "garrisoned" (by 5, 85, or 3,000), but would a single officer from the Corps of Engineers constitute a garrison?. With your military experience, if the CiC asked you to garrison a position, how many men would be placed?
As many as it took. But if your definition of garrison is an armed body of men defending the fort then no, it wasn't garrisoned until December 1860. But the construction efforts were ongoing under the supervision of the Army engineers. Now, what's your point?