I have personally hired false positives. Like you say, they are expensive. I have never once hired someone who was literally incapable of doing the job. It's never because of work directly. A person who was incredibly professional in the interview turned into a massive asshole at the office. One fought good coding practices because he thought it was over-engineering. Another spent all day playing online poker on their phone. One spent their entire time at the office making Forth interpreters.

Doing 7 hours of whiteboarding interviews for every candidate won't flush out any of these. Not a single one. If you can tell me the magic 8 ball I need to shake to fix this, I am wide open to suggestions.