A valve cover gasket leak will not leak at the rate you are mentioning, you're correct. The oil filter adapter can however, and if the valve cover gaskets were leaking I would recommend them at the same time too.
However, when you have a big leak, and several leaks (possibly) it is important that you do what they did the second time and that means cleaning and adding dye. The mechanic was negligent in that they didn't properly diagnose the problem, I would say they were trying to rip you off, they just were lazy and didn't take the time to properly diagnose the problem.
You did the correct thing by sending the owner a letter, see what happens. You have other options too if they are non responsive, but give them an opportunity first to address the problem.