As someone pointed out to me a while ago (was it you maybe?) there's no guarantee Nika's dream matches the events of reality. He might not be reliving the evens, his mind could be plastering more digestible fantasies over the disturbing memories like a bandaid.
From what Nika says to Amber and Nami we can deduce that there was an officer, but the dream itself and his flashbacks aren't reliable.
So an idea - Nika and Summer disappeared, Nami alerted Officer Larverne, Larverne took them back to Summer's parent, Summer's parents filed charges against Nika for kidnapping or something. Then the whole Summer clan (whatever their family name is) disappeared. ie the parents are gone too (otherwise it would be the easiest thing for Elsa to know what happened, no melancholy photos on aurora about the past, Nia could find out what happened to her only friend etc).
The violent incident happens after NIka gets reckless from losing Summer, which means the violent incident could be as a result of Nami, and the way she was teasing boys to get Nika's attention. All that happened after SUmmer disappeared, and that's when Nika stopped playing basketball because fights made him feel alive. So the violent incident happens after his violent period. Maybe he gets charged then instead? But he gets hospitalised. So initially, he's not depressed so much as violent from unhappiness. The depression comes after being hospitalised, after whatever Nami did - this perios he talks Amber about anyway, so we can kind of extrapolate.
This would link in with the idea that NIka says Nami is the reason he is the way he is.
As far as Summer - the fact that Nia doesn't know what happened to Summer either shows that no one knows, which further suggests Summer's parents aren't around to answer questions.
Sorry, this is poorly explained, and circular, but you get the ideas I hope
