Being 15 behind on the event doesn't matter because like Aidoru said, the devs stated doing every quest will give you full event completion by the end anyway.
Also most of the girls require energy drinks on affection rank up requirements anyway, so 5 isn't that much. In what I've done so far, Cleo and Sasaki have needed 2 each, which I think only costed 50 gems. And if you're not doing affection rank ups, just get 5 coffees and it will only be 50.