I think Summer is just a trigger to the story. The story needed a traumatic event for the MC to explain is awkwardness, his libido, the tension with is "family" and a shared link with Bella. We will probably never know what happened to her until the end of the story, if we ever learn about it. I think the story will feel more real if we don't know about it, because that what happen in real life. Sometime you need to lear to go forward without knowing what happened. This way, the dev will not have to deal with high expectation toward her character and the story will be one of healing for the MC without an easy way out. Bringing her now(next 2-3 updates) in the story will kill the mystery vibe of the game. The dev will need to come up with an incredible story to make it believable that she never tried to contact the MC before or anyone around him...Her being in coma or amnesia is not a great way of doing it, it's the lazy way of doing it.
I'll do a parallel to the TV show "The leftovers", the whole trigger to the story is never explain until the last(or penultimate) episode of the last season and to me that was the best decision they could have done. It was only something needed for setting up the context of the show, not having an explanation and not focusing on it, put all the attention on the reaction of the people to that event.