Don:

Reading Petrie you will find the core stones form four sides with greater accuracy than the casing stones, also, the core stones have a uniform "twist"/orientation compared to those of the casing.
This suggests the core stones set in place before casing stones were introduced.
Following the casing stones were the abutting paving stones.
The corner stones were set into holes dug into the rock base termed "corner sockets".
Each socket accepted a single casing stone that had to be pre-finished then set in place. A 4-6 inch space between the base of the socket edge and that of thee casing stone allowed for final adjustment and alignment.
It was once believed that the outer limit of the corner socket was the outer extent of the pyramid side, but these socket vary considerably in depth and width bringing to the attention of Petrie who abolished the theory after realizing that the pavement stones were at a uniform level at all points of the perimeter compared to the sockets (+/- 19 inches in depth).
His belief that the pavement being the intended reference of measure proved correct, producing four side measures having greater accuracy than the four outer socket corners.

Your question as to when the lower casing was set in place...be it at the start of construction, partial or at completion does not appear relevant to the finished product.
If I were overseeing the project, and there only being one course of angular stones used, then I would set them in place after the apex was completed...the corner stones were the finishing touch that held several additional measures for us to analyze.

Best.
Clive