One thing I just thought about was that a big problem could be the number of doctors Lubitz went to.
This way he spread out the crazy to several doctors, enough that noone could see just how much crazy he had. Although some of them did acknowledge the crazy and wrote him prescriptions and sick leave, they were not entirely in the loop. It was only after-the-fact when all doctor notes had been collected that the total crazy became evident.
Is there any airline that has an appointed psychiatrist for crew members? This, including job security, could reduce risks much more than putting a CC member in the jumpseat. IMHO of course. And SLF status behind the keyboard, I might add.