(Side note: does the "core memory unlocked" here imply that Sensei now remembers raping Molly?)
I think the core memory being unlocked actually refers to him ‘remembering’ more of Sekai, AKA his suppressed memories of her are beginning to loosen.
Sensei talks about the smell of Spring and cherry blossoms that same scene if I’m not mistaken, and we know who cherry blossoms represent.
Furthermore, and I may be reaching here, take a look at the position Sensei and Io are in here, then compare it to the position he and Sekai are in in the “Mother’s Milk” Happy Event when she jerks him off. Perhaps that’s the ‘memory’ in question.