I love the connection you drew between echolalia and idioms. I think that part of my masking involves frequently having some quote from something be the first thing that pops in my head in response to something and then filtering it in my head into something that doesn't sound scripted when it gets revealed to the outside world. I'm so good at it that only one person, a therapist who specialized in later diagnosed autism and ADHD, even remotely picked up on it, and that was about two years into seeing him every week. He made me realize that I can use the echolalic thought material to better access my emotions. I also have a tendency to have very specific, almost synesthetic, highly literal visualizations of idioms, which I then map to my running stockpile of memorized meanings.