Oil pump was torqued properly. I must have installed at least 2 dozen over the last 35 years.

Pump never completely seized. It is very hard to turn by hand (with an intermediate shaft/gear, even with the outer rotor removed). Oil pressure drop may have been due to the main bearing failing and excessive clearance. So, it may have been binding up on it own due to poor manufacturing, due to residue from a bearing failure or who knows what else

Try taking the center shaft out and put it in a drill press to see if it't bent.