I don't think the psychotricorder scan would have any lie-detecting abilities - it is not credited with such, or handled in a way consistent with taking a testimony and scrutinizing it.
Rather, the device probably does what it was credited with - it tells whether Scotty really has amnesia or other such "psychological trauma". If he does not, them he's already caught in his first lie, no truth serum or polygraph required. But our heroes apparently hope the device will show damage and help make Scotty less responsible for the crimes, regardless of whether he committed them or not. Losing this opportunity is no big deal, not when Scotty gets tangled up in yet another murder, so there is no real point in sending down a second specialist to run the device.
Finding out whether Scotty is insane is likely to warrant privacy. Depending heavily on the result, a revelation can then be made, or not. Any guilt-supporting result would and should be withheld from Hengist, unless the local law works differently and self-incrimination is mandatory. Hengist can't really complain. But he would if the heroes tried to move Scotty to the ship, on apparent escape attempt!
Timo Saloniemi