Should Juvederm Be Injected Directly into Marionette Lines?

You and your doctor injector are correct. It is a three dimensional injection, into the valley or crease and along the edges also. Maybe you should see another opinion of an expert injector. Regards.

You're both correct. Juvederm should be injected into the marionette lines (at the appropriate dermal depth) for improvement, but it also needs to be injected beside and often across the marionette lines for more substantial support.
Different injectors have different techniques, and advanced injectors will place product both into the lines and around the folds to achieve the best results. 6 months is a pretty typical duration for injections around the mouth, though. Juvederm can last longer (if enough is allowed by the patient and given by the injector) in other parts of the face, but the mouth is such a mobile area, the results don't typically last as long there.