I agree that the whole Nitrox thing works on blind faith. The sticker is meaningless. Heck, I have the certifications from PSI to inspect and clean my own tanks and valves. I can 'self-certify'.
A viz around here costs $7. I don't know what an O2 clean costs but it isn't much. In fact, I won't even bother to inspect my own tanks because it is so cheap that it isn't worth the hassle of buying stickers.
I tend to treat my tanks with care and, so far, only the LDS has filled them. When we are diving at a location that requires local filling, I rent shop tanks and dive on air. But I don't contaminate my own.
When it comes time to have the tanks inspected again, I expect nothing less than perfect interiors. They were bought new, filled only at the LDS with either Nitrox or O2 clean air and there better not be any rust.
I would go with the flow in terms of tank inspection and cleaning. I would allow them to inspect my personal equipment but I would not expect anything to need service. Even though it is the student's equipment, the shop still has some liability if something goes wrong. The survivors' lawyers are bound to ask "Why didn't you check his equipment?".
If the inspection did uncover issues, I would simply say that I'll bring it back next time. I absolutely wouldn't have that shop do the work. Even if I had to drive 100 miles out of my way. So, get the first inspection early enough that there is time for a retest.
Richard