It is absolutely possible for a non-DOO train to operate with a Driver and Guard and also have a ticket inspector/revenue protection officer present. I don't know if Scotrail do this, but certainly other TOCs do.
Scotrail did. Peak time EK services used to have a conductor dealing with doors etc and sometimes extra ticket examiners who collected fares - mainly because the conductor had no chance of getting through the 6 cars for fares with frequent stops.