Back with another problem for the altered note:
Managed to get the note. Took it to Cassidy and got her to give me the numbers. Then ADHD kicked in and I started power-levelling Alex all the way to pregnancy. Remembered the note and tried to get back on track: now Hailey doesn't meet me friday night anymore.
I saw that the event has requirements to start: Haley C3, Alex/Lina/Sami C2, Jenny C1 (which don't make sense given Lina's C1 requirement to alter the note in the first place) but matching the girls to them doesn't trigger it. I even tried looking back at the previous save to match the corruptions exactly, but no dice.
I think this may be a bug??
Update: did some testing and the only change that is making the event trigger again is Lina C2. Managed to brute-force every hint except "with Lina's help... alter the note..." but I think that's where Lina C2 is stopping me because when I bring the note to Lina and Jenny Sunday noon/afternoon (the only time after actually getting the note when I can talk to Lina) and bring up the "cryptic note" option, it doesn't tick off the hint so I don't think that's it.
I see. Just to be clear, the events that have "EVER" in them are permanent events. So you can trigger them and they will always be crossed out in future cycles. Also, Lina being C3 will work instead of having her at C1.
The prompt you're looking for on Sunday: Noon is "Borrow Lina's Camera". This is triggered by a condition called The Note's Secret. Hovering over it will give you a description on how to activate it.