After Ben (as Liam O'Connor) cannot give his name, the current president, or the date, the doctor diagnoses him with short-term memory loss. Short term memory is the brain's ability to hold onto information for a matter of seconds, usually necessary for a task at hand (for example, remembering a phone number someone just heard, or remembering why they walked into a room). Being unable to remember one's own name or the current president would be a very different issue.
Sodium thiopental does not actually work as a truth serum. It is a barbiturate. While it can sometimes lower inhibitions, the idea of it as a functional "truth serum" is largely an invention of Hollywood, and the "confessions" attained by its use in real life are highly suspect.
At 40 23, Janis steps on Magic's phone, cracking it and explaining that she's worried about someone listening in. Cracking the screen would not reliably stop the phone from working.
If Liam O'Connor was a Private Investigator it seems odd that he would use his real name to get into the asylum. It might have been more difficult for the people in charge to figure out who he was not impossible.