"My KP-15 bolt catch or PDQ is locking open with rounds in the magazine"
This is a symptom of excessive bolt speed. The bolt catch is bouncing up as the recoil cycle happens.
Follow our buffer system guide here to slow down your bolt.
Or use 2-4 coils of another bolt catch spring underneath the main spring to increase tension. Put the cut end facing down and make the flats touch each other.