This is just me typing out loud here.... But let's think about Dassey's confession. He was 16 years old at the time; if he was there while the murder was taking place, I don't care who you are, IQ level, whatever. That's a traumatizing experience. Especially as a minor. Is it possible that Brendan subconsciously blocked the memories? Could he have created this fantasy in his mind of shooting her in the bedroom, garage, wherever, and also doing terrible things to Teresa's body, because he doesn't want to remember the truth?