Senate Bill 420 legalized medical marijuana in California.  This connects 420 and marijuana, indicating a possible origin of the modern term. Settling where 420 originates from would settle a decades-old cultural debate.
California Senate Bill 420 is the Medical Marijuana Program Act, which legalized medical marijuana in California in 2003.  This bill provides a concrete example of a strong link between marijuana and the concept of 420, strongly indicating that the bill created this term. 420 was formalized as early as 2003. The phrase lacks a distinct origin because it's drug slang that didn't originate in popular culture. Even if they coined the term, there is no single person who can claim to have created 420. This is because 420 as a thing was created by exponential numbers of people using it in group or social situations to spread it around. Therefore, 420 was originally created by all its users, but the 2003 bill gave birth to a strong, public link between marijuana and 420, thereby creating 420 as it exists today. By taking 420 out of the public lexicon and formalizing the link between marijuana and 420, the bill officially created 420.
Although California Senate Bill 420 may have provided an official, public link between marijuana and 420, the bill did not create it. 420 is a piece of slang that has been used in its modern usage since the 1970’s. The CA Senate Bill is from 2003.  420 has contested origins, but it is definitive that nothing so recent created it. Furthermore, it is more likely that the legislators of bill 420 were aware that 420 was a celebration of marijuana, and therefore used that number for the bill as a sort of harmless, quiet joke.