I think maybe the point is being missed here - it's not just the fact that the HSTs are needed for passenger service, it's also that there just aren't any spare crews to deliver the training anyway. All of us that are Driver Instructors have Trainee Drivers - they work to the instructors shifts/diagrams. If the instructor is pulled away to work at the practical handling side of HST training, not only is there a Trainee Driver who's having their training impacted, there's 2 weeks where a Driver Instructor's shifts need to be covered. Factor in the requirement to have annual leave/sickness/lieuday cover, then maybe you can see the issue.
There should be enough drivers trained on HSTs (in Inverness anyway) to deliver the timetable. There's a separate 'non HST' roster for the small no. of drivers who haven't been trained yet. All the Conductors (as far as I'm aware) are HST trained now.