1-Perhaps he was just in possession of some dank weed.
2-Valid question. There had to be some probable cause.
3-Yes, lack of DWAI (ability impaired) would signify the patrolman believed him not to be stoned.
I'm curious to was the PC was for the cop to pull him over. Was he speeding, swerving, tints, loud music, was it something other than any of these that made the cop think something was weird.