I work at CK and I'm of similar opinion to yourself in many respect.
All my students get on the plane ready to leave it as do I. No straps need to be tightened or adjusted in the plane. If I'm turning which I rarely do out of choice - I will check the student and my own harness prior to climbing on plane. No exceptions.
Other staff do leave the harnesses a little slacker (there personal choice/style) but I dont see any students getting on plane with harnesses that slack they need to be tightened up drastically before leaving. Simply a comfort factor thing.
CK in summer is busy - we have people in hanger specifically gearing students up ( hanger master ).
The are briefed by an instructor as a group when gearing up and personally I reconfirm details with all students prior to jumping with them. As I check the harness is adjusted to my liking.
It is ultimately the instructors responsibility to ensure the students harness is fitted prior to jumping.
I have jumped for over 13 years and have travelled to DZ's around the world. Cross Keys is busy in the summer but does nothing that would give me cause for concern. Sure spending half an hour with students would be great but commercial reality is that most people are not coming to get into the sport - just for a once in a lifetime thrill ride experience.
The staff and procedures at Cross Keys cater to that in professional manner. Literally thousands of tandem jumps made without incident are probably testement to that.