What about the issue with Alex's missing unlock messages/mail from the cheat menu, unlocking Haily's messages/mail with the cheat menu leaves the last two messages/mail locked, unlocking Jenny's messages/mail with the cheat menu leaves the last message/mail locked, unlocking Lin's messages/mail with the cheat menu leaves the last two messages/mail locked, and unlocking Lina's messages/mail with the cheat menu leaves the "Cum inside Lina while she is fertile and completing her Corruption 5" aka LinaSMS21 ":)" message. I also was curious and checked your rpyc files and learned some things. I think it hints Jenny has a corruption level called Devourer and Maiden. And finally, Lin, Cassidy, and to a degree, Lisa, I know we just had a poll but the fact that they still only have 1-2 corruption levels, while Sami, Alex, and Haily, have 8 is shocking.
The cheat menu is hard coded and I don't always remember to update it... It's not normally a priority as the things should™ be accessible in game.
Jenny does have those corruption levels, with her C4 being another split level.
As for some characters being left behind in levels. Lin is already completely realized as a character, and she's not a perfect woman, but she doesn't really have much more room to grow... emotionally I mean, she will obviously be getting pregnant.
Lisa and Cassidy have been lagging behind because they aren't chosen in polls very often. This compared to Haily, who has won every poll she has been in and rapidly rushed to completion.
I have an interesting idea. Similar to the "?" function, you could include a function to be an unlocking mail guide. The icon could be a ✉, be above or below the scene map button and have the same character icons on the left of the screen as the "?" layout. If you want to unlock a specific mail like Alex's "Clothes that 'Fit'" clicking the mail lock icon will say "Tracking mail", the map will have a ✉ on the "Forest with Alex" square by the red and purple corruption points small circles. In the "Forest with Alex" section it will have an ✉ on "Talk Normally" then "Why aren't your pants done up?" If it is an already unlocked mail it will be 
The picture shows that at noon on Saturday I selected "Forest with Alex" and unlocked one of the two available mails