All labs are apparently corrupt and fradulent
I doubt that forensic labs have KPIs tied to the percentage of positive tests. Drug tests are often mandatory in various criminal proceedings, and the lab gets paid for a test performed, not for particular result.
If they follow good practices, all samples are blinded, and fabricating results can backfire spectacularly in court. In Fentanyl Floyd's case, you can't blind a dead nigger's body, coroner will always know who's laying on his table. So Floyd's bloodworks (that came from the CORRUPT BIG PHARMA-FUNDED LAB) weren't tampered with, only his epicrisis.
Attacking the hair follicle test
Follicle test isn't useful for testing truckers because there's no way to deduce when the drug was taken, drugs remain in follicles for many weeks. If a trucker blew some coke on his day off, it won't affect his ability to drive and shouldn't concern his employer. That's why an additional blood/urine test is required and that's why it often returns negative, because drugs have already been eliminated from the system by that moment.
One can argue that cutoff value for ELISA is arbitrary and values close to cutoff are meaningless. Then again, Nick's daughter tested 10x the cutoff.
As for external contamination argument: I don't know if the specific test they used is highly selective for a certain molecule (it probably is since it's ELISA), or does it have affinity for all "family" of cocaine and its metabolites. Again, we need to know more about the assay. And if the lab still have the sample of hair (as they should), they can take it to a different lab and run different type of test, like quantitative GC/LC-MS to get a full profile of metabolites and rule out contamination.
Too many generalized statements on Barnes' side, good for grifting, bad for building actual defense upon them.