Well, so far you're all on target.
The drop ceiling folks do include specifications for minimum space above the grid - I think it's 5" - in their instructions; but, as has been noted, we're talking about ceilings installed by DIY's and other unprofessional folks.
There is at least one troffer made that is no thicker than the grid - call it 1-1/4".
I was referring to the lenses sold at the home stores fitting in the grid, but your needing to trim them to use them as a replacement lens for a troffer.
The two places I tend to see these are basements - to hide all the ceiling clutter - and kitchens.
The strip fixtures are invariably 'cheap, cheap, cheap.' Cheap even by strip fixture prices. Yes, their install often violates the instructions (some call for a clearance, to not be flat against the ceiling), and have flexible cord connections (a question of 'when' as to that being a code violation).
I didn't want to get bogged down in all those details. It appears that there is no code rule against the basic idea of using strip lights and independent lenses.