I think the question we should be asking about the check marks isn't "who does each checkmark represent," but "who is putting them there?"
Akira doesn't seem to be interacting with the corkboard at all. When Maya talks about it, he seems only vaguely aware of its existence, so I doubt he's putting notches on his belt, so to speak. Someone else is putting those there, and we're only assuming they're representing girls because of the "bonus" check mark if you're fucking Ami.
But who? And why?
My first guess -- pulled directly out of my ass -- is that they're progress milestones on Akira's journey to recover himself. I'd need to replay the game from scratch to develop this idea further, however, because I'm a year removed from my last run through the early game, and my shit memory is gonna get things out of sequence (I do not have time for this right now)
Second guess -- also pulled out of my ass is that they're things that have to happen before Nozomu can return. He's certainly more aggressive this summer than he was in the first summer, indicating that something changed, even beofre Sekai returned, so what was it that caused that to happen? Again, this would require a thorough examination of the first two chapters to figure out.
Anyhow, my point is that the checks = girls might be a red herring, and they could mean something else entirely. If I have time once I'm caught up on tomorrow's update, I'll pore through the Ami lust scenes and see if there's something that happens in one of them independent of the sex itself to see if maybe that "bonus" check means something other than simply fucking Ami.
