Strictly speaking, Persona 4 is an RPG with some galge elements in it, such as developing Social Links with various characters, of which some can become romantic relationships. Also, I know that he probably has never played it by virtue that he is a fictional character and would not have the patience to go through an 80-120 hour game.Is Persona 4 considered a galgame? If not, I won't be surprised if Keima never bothered... Theres way too many galgames for him already. That and he would play single routes anyway. Even now, he is technically playing single routes with each of the girls, so its not harem route YET. Just consider him playing multiple galgames at a time for now, at least until the human girls find out.
As for all of them gathering together to kill Keima, it seems hard, since the goddesses all gather to fight for, and protect him, despite seeing how he is charming their sisters. And the goddesses seem to reflect their human counterparts or vice versa? So even if the girls find out, they might take the same routes as the goddesses did when they met, or just act angry while still being in love with Keima. I mean, all the goddesses made their appearances(meaning none of them has lost their powers from "Love", this is the case even for Ayumi, who knew that Keima is going after her best friend).
On to your next point, while it seems that the sisters are more emotionally involved and reflect their human counterparts more than they probably should, that does not mean that Keima can keep acting like a clueless jerk all of the time and get away with it. They are going to have to wise up sooner or later. Keima truly is playing Russian Roulette here, but I get the feeling that he is going to pick up the semi-automatic laying next to the revolver if this keeps up.
