This Audi owning retired Ford engineer has made an extensive study of carbon build up on intake valves. He agrees that the PCV valve is the primary culprit and offers cures such as dumping blow-by into the exhaust . He bought a pricey book that concludes the valves are no longer susceptible to carbon build-up once they reach 280 C.
http://www.audiworld.com/forums/a6-s6-c6-platform-discussion-58/carbon-buildup-cel-tsb-update-2777956/

Seems to me that, if blow-by is the culprit, every direct injection engine w/o extra port injectors or Ford's overlap system is doomed to disassembly and cleaning at increasingly frequent intervals as blow-by always increases with age. Wonder how expensive that would be for a ML.