You hit the nail on the head. The 18-hour rule seems to be intended to be interpreted in the context of a vocational study. There is no specific interpretation, as far as I know, that confirms or denies that the rule is also applicable to flight training. And even if so, there is no list of stuff that is or is not to be included in the 18 hours. Actual flight time, pre- and post flight briefings, flight preparation, theory study, exams?
If you look at the intent of the visa regulations you can conclude that if your primary reason of visiting the US is flight training, you should have an M-1 visa. But since an M-1 visa can only be sponsored by a SEVIS-approved school, and only Part-141 schools are SEVIS approved, this leaves a lot of students in a bit of a fix that want to go to a Part-61 school.
I think you've got two options. First is to contact the US authorities, explain you want to take a specific course at this-and-that Part-61 school, and ask them how to proceed. Second option is just to enter under the VWP as a generic tourist, and hope for the best.
In any case, this is totally independent of whether TSA clearance is required.