I was looking at the source a bit earlier, trying to see why getting Deep Seated Illnesses (permanent nympho, obsession, volatile) was so damn hard. And this is what I found (assuming you already have a temporary illness from Psychotic Break):
1.) If her willpower is over 50, it's impossible to convert a Temporary Illness to a Deep Seated Illness.
2.) If her willpower is below 50, there is a 1 in 16 chance of it triggering when she's exposed to Insanity Aura (everytime her sprite appears on screen, and casting 99% of Mental abilities causes it to trigger again, like Lust Overload or Mindcraft)
3.) If she already has a Deep Seated Illness, there is a 1 in 16 chance it will be converted to a Temporary Illness. ("permanent" is a lie and Deep Seated Illnesses don't require an identity wipe to remove)
4.) There is a 1 in 8 chance that the girl will randomly change her Temporary Illness (from Nympho to Obsession, for example). The code doesn't bar this working on a Deep Seated Illness, either.
5.) There is NO stat requirement outside of Willpower being below 50. All this information on the last 300 pages about love/lust/corruption only applies to the initial Temporary Illness, but it has no bearing on getting the Deep Seated version.
Below is that source. The first line ("$ var2=renpy.random.randint(1, 16)") tells the engine to roll a 16 sided dice and set the resulting number to 'var2'. The next 16 lines say what the game is supposed to do if var2 is 16, 1, 2, 3, etc ("if var2==13:").
16 = change a Deep Seated Illness (char.psylocked) to a Temporary one (char.unstable) or make the Temporary one Deep Seated.
1 = change to Volatile, or make Volatile a Temporary Illness if no illness exist, no incest
2 = change to Obsessive, or make Obsessive a Temporary Illness if no illness exist, also permits incest.
3 = change to Nympho, or make Nympho a Temporary Illness if no illness exist, also permits incest.
4-15 = apply random stat changes "due to Insanity Aura"
... it's completely doing shit at random. Literally. As long as she has less than 50 willpower.
And then I got what I was after.