SOLVED! [ Reinstall without the VIET Events module (i.e., type 'n' when prompted about it in the installer). ]
Yep, and sure enough, as soon as I started a game with VIET Events enabled, a child character cropped up with the known_murderer character_modifier on 1066.11.11 (Stamford Bridge bookmark, so it had about 5.5 weeks before it happened, as opposed to the 110 years of it never happening w/ VIET Events disabled). Therefore, the quick fix for the 'Known Murderer bug' is to disable VIET Events for now.
And it does look like my initial hypothesis about a mistake in VIET's 00_on_actions.txt may be true: event ID 10 was invoked on a child, but it was not invoked from any of the non-VIET places (including all of vanilla) that invoke event ID 10. The problem lies in VIET.
Yep, and sure enough, as soon as I started a game with VIET Events enabled, a child character cropped up with the known_murderer character_modifier on 1066.11.11 (Stamford Bridge bookmark, so it had about 5.5 weeks before it happened, as opposed to the 110 years of it never happening w/ VIET Events disabled). Therefore, the quick fix for the 'Known Murderer bug' is to disable VIET Events for now.
And it does look like my initial hypothesis about a mistake in VIET's 00_on_actions.txt may be true: event ID 10 was invoked on a child, but it was not invoked from any of the non-VIET places (including all of vanilla) that invoke event ID 10. The problem lies in VIET.