You are conflating Nika's dream at the beginning with his bona fide memories in the flashbacks.If Nika's dreams and flashbacks/memories aren't reliable then whatever he tells Amber during therapy isn't either and "fixing" Nika will be harder than anticipated.
Nika can't control his dreams, and you should know like the rest of us than dreams can contain some real funky shit that bear no resemblance to reality, past or present.
So, no one is saying Nika's memories are unreliable, other than Nika not telling Amber the whole truth yet during therapy.
There are fantastical elements in Nika's dream, plus it seems to present a reality where two kids casually walk to a cabin six-hours drive away by car (this latter point could be a plot hole, we just don't know yet). It's future therapy with Amber that will likely provide a more factual context to everything that happened during the "cabin incident".