In 0.904/5.3 I can generate Arrogance Subjugated just fine with regular scoutings after the first time.
tl;dr tangent - Just keep Scouting:
The way assignment generation works is that every location has a list of assignments it can have, scouting them reveals them and once something has been revealed, it can't be generated again.
This means if you want a specific one (QAYL, RotWQ, or Arrogance Subjugated) running multiple scouting missions the same location before assignments expires is helpful.
You can also thin the pool by completing the unique assignments, although the effect of this is much less than just running more scouting missions.
4 crits is ~20 missions, which is not enough to guarantee a generation of any event in any location.
And True Angels CAN be turned into slavers through the possession corruption.