Took me ages but I finally unlocked everything.
To unlock the last event (and the 6th CG at the same time), you have to ... (spoiler warning)
...
Tell Si-Won the truth
Do all the sex-related stuff, take Si-Won up on her offer
Tell MalO to keep blocking Zellen's data transfer and release her
Go talk to Zellen
Be honest about your relationship with MalO