It is my understanding that the problem with the clear is "delaminization", where the clear is coming off of the color coat. Anything short of striping the paint and repainting, from what I have seen, will fail in a short time. The clear is coming off because of poor adhesion to the color coat. They stick together because of a chemical reaction. That is why the manufacturers have certain times specified between the coats. You cannot sand off the clear coat and not sand into the color coat to get a proper adhesion without sanding the color coat thin in spots, or leaving some of the "bad" clear on there to peel later.
Can you shoot clear over an old paint job that has no clear on it and get a good finish that will last? Possibly, if done right, with the right products. Can you shoot it over a bad finish of BC/CC, and expect to get a good finish that will last. NO!
Just my opinion, from experience. I only recommend what I have seen that works. Not what I have seen in pictures.