There will be buildup eventually due to the direct injection method of most vw and some other engines. More so related to the petrol rather than the oil I think. From what I understand, marvel mystery oil works to different degrees of success so beware as results may vary and for the price of one, not too sure if it's worth it for every oil change, but really depends on how you drive the car and what gas you use. I typically recommend a higher octane rating at the pump like 91 or 93 for direct injection even if unspecified by manufacture cause the gas would technically be cleaner than the 87 or 89 stuff. If you go to a cheaper gas station find a good fuel additive every once in a while and you should be having not to many problems as long as you don't thrash the car everyday, look of for snake oil too on that note.

No it wouldn't because there's no oil there. You would need to use an intake cleaner system, but I am skeptical of those as well. If I wanted to be sure the valves are clean, I would have the heads disassembled and physically scrubbed. Or at least get a boroscope camera in there to see how they look.