That event and CG are unlocked in the same way. I will copy the process I put in a reply farther below.
I am not sure about the diamonds. I guess they stand for the endings? I have them all unlocked but never paid attention to them until you mentioned them here to be honest.
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