I'm a bad person, I haven't posted in ages.
I don't think the 3rd witness would be Amy. I think Dean feels guilt about lying to Sam but not about killing Amy, he feels he did the right thing there. Dean was doing what he always does, saving people and protecting Sam. Even if it's protecting Sam from himself. If Amy slipped again then those deaths would be on Sam for letting Amy go.
I would have loved the 3rd witness to have been Cas but I live in hope that he's out there somewhere so I'm glad it wasn't him. I like to think it would have been Jimmy.