Good question. I opened up my long-running save and somehow I was only at three (even though Lina's first corruption level requires a nerve of 4?). After a few rounds of redoing, I've found seven points (this may not be in a useful order):
- Biting Jenny on Sunday night after getting her to 2+ love and priming her as prey. (May be safest to have her a 0 corruption when you do this.)
- Saturday horror movie with Sami at corruption < 2 and love >=2: "massage her feet," "tickle her," then "put up with it."
- With Jenny set to "corruption: prey," go to the Saturday horror movie with Jenny, say, "I liked all the sex and nudity," then kiss Jenny.
- Sunday morning, Lin's bedroom with Lina corruption 1: "How do I get along with Lina," and keep on saying "Okay" until done.
- Friday visit to Alex at corruption 4+ (but no pregnancy) and Lina at corruption 1+: "finish what she started"; "Lina doesn't mind."
- With Lina corruption 1+: on Saturday afternoon, go to Lina and Jenny, and see what Lina's fiddling with. "Let it be."
- Same as 6, but do it at corruption 4+; then you'll take a group photo at dinner.
I thought I'd checked all of the scenarios that Lina appears in in the in-game guide, but I suppose I must have missed something (and at a lower Lina corruption level, given that only three of those appear to be available at a Lina corruption of 0). Hopefully someone else points me to the missing one.