i'll only start the new update literally after posting this reply, so take my words with a grain of salt, but
while your theory sounds highly interesting, i'm still more convinced that the whole 'mina is not doing the bucket list with ian but now with mc' is more playing into that ultimate nice guy fantasy that a girl will only do the real naughty stuff with you if she 'feels safe' with you and 'trusts you fully because you don't cheat' (behind her back)
would love to see your idea being whats really going on though lol
Trust is definitely important but then, how has the MC built up trust with Mina when he's literally going behind his friends back with her? Why would you decide to trust someone who betrays their best friend like that? I mean, Killian was pretty obvious with his cheating on Mina and when the MC asks why he left the sex video around for Mina to find, Killian can't explain why he did it. Personally I think he did it because he has a self-sabotaging personality - whenever something is going good for him he can't help but fuck it up by pushing it just that little bit too far.
I'll give you another reason why I reckon Mina's plan was to always cuck Killian - she says that her mother is not very 'sex-positive' and views getting an education for a girl as a means to marrying into a 'well-to-do' family. Meaning that Mina has been brought up to be a very 'tradwife' type of girl. She even says that her mother would love the fact that she and Killian were together - her mother was Mrs. Eiko Adams, then Mrs. Eiko Scott, then Mrs. Eiko Umeda-Wright. I'm guessing her maiden name was Eiko Umeda. This shows what kind of woman Mina's mother is - Mina says that they 'butt heads sometimes over what's traditional' and that she has a 'generous heart' that 'maybe a little too generous when it comes to the men' - implying that Mina's mother is a bit of a doormat.
Mina, however, doesn't view herself as a trophy to be won or a doormat to be stepped on. Her upbringing has made her very repressed sexually to the point where she's actively into things like foniphilia (if you say that using women as an object is the MCs fantasy during the game on his birthday, it's the only choice where she smiles and gets flustered).
Ask yourself what would be a bigger fuck you to her mother's opinion on what a woman should be than to reject the guy who her mother would consider to be a great catch by cucking him with his best friend who is studying to be a doctor, who Mina considers to be a great catch?
I just think Mina is more manipulative and smarter than most people give her credit for.