Yeah, they're just fan theories. Besides, even if, say, Ocean had originally planned (and I am not saying he did) such a thing, it doesn't mean it survived the reworks.
A dark Nami option would be: she orchestrated a sexual experience with the MC that Summer discovered. Where else would the MC get his idea to destroy the Holgersons by orchestrating an infidelity? His own experience.... As I said, 'just fan theories/musings and I don't take it seriously.
All we really know on this matter is:
- Something happened between Nami and the MC at an unspecified time. The MC states Nami is partly responsible for the way he is and what happened is the reason for his lack of impulse control (I can't quite fathom what impulse control the MC lacks, unless it's his propensity for fighting). MC/Nami made a deal "a long time ago" to bury the incident.
-MC accuses Nami that all she does (now) is an overcompensation to make up for what happened.
-Nami tells MC she would never be "like that" again.
-MC says he lied to Amber during his sessions by altering details he told her (this could be about Summer or about the MC "protecting" Nami at High School).
-MC says he needs to tell Amber (breaking the MC/Nami deal), to come clean with himself. Nami tells him not to come to a "weird conclusion" because of it.
It's all very cryptic, obviously. Hence why it's easy to wildly speculate.... Perhaps it's something as simple as... Nami deliberately flirting with boys at High School to make the MC jealous.