Amelia is not real.
She is either a coping mechanism or implanted memories.
- Her name. It’s AMELIA. As in a form of Amy. As is Amy Pond. There is no reason why they would name her such a similar name if there was no connection. THEY EVEN RANDOMLY THREW HER NAME INTO THIS PAST EPISODE. In this way, she could be a representation of what Sam wants.
- Amelia is SO SIMILAR to Sam, it’s kinda creepy. She sounds almost as if she’s repeating Sam’s life back to him. Don. Dean. also similar names. In this way, she’s like an extension of Sam himself.
- COLOR TIMING. This is a filmmaking thing, but over-saturated footage (especially like in his birthday scene, which included a bloom effect to boot) usually means a false nostalgia and sentimentality. You know, making something appear better than it is. In this way, it could mean that these memories are covering up something worse. Maybe Sam’s wall isn’t fully healed. Maybe he was tortured by Crowley or someone.
- Also, the quantity and random nature of the flashbacks themselves. They are brought on by general and vague stimuli. In cases of implanted memories/coping mechanisms (at least in literature and film), the triggers of the fake memories are general so as the reoccur as many times as possible to fortify the memory in the implantee’s brain.
- UPDATED THEORY: Naomi did it. Castiel AND Dean now both are evidence of having false memories. Naomi did it to Cas, perhaps to Dean, so why not Sam?
- UPDATE #2: Samadriel, while possibly just a clever mix of Sam, Dean & Cas’s names, is very close to the angel name Samandiriel, the angel of imagination and the way we perceive the world. Sounds like he might be known for futzing with people’s minds, although I have no motive for him to want to do this.