What's worth noting about the "accident" story is, the actual number of accidents didn't change AFAIU, what changed was how many people involved in the accidents had detectable THC.
Again, THC which can be detected 3 weeks after smoking. No one is seriously trying to suggest that people are impaired 3 weeks after smoking a joint.
What's different, now, is that marijuana is legal, so perhaps there is a greater percentage of the population using it recreationally, and thus a greater percentage walking around with detectable levels even long after not being impaired. Some percentage of those people, unfortunately, get into fatal car accidents, just as they did before.
It's like saying "the number of people involved in fatal accidents who also have a facebook account, skyrocketed from 2006-2016!" The implication that the facebook accounts caused the car crashes is fallacious.
As far as pesticides in cannabis, the best argument against that is legalization.
Voila: Legalization and regulation in action-
https://www.oregon.gov/ODA/programs/Pesticides/Pages/CannabisPesticides.aspx