I do think Josh's verbal/mental decline is is most affected 2 things. First in those older videos I am pretty sure he was still on ADHD medication (I may be wrong but taking ADHD medication while not having ADHD can cause ADHD like symptoms like hyperactivity) - so it may be that Josh was always somewhat slow speech wise but the medication made up for it. And second, the spice abuse during the bloated river corpse saga many years ago. No longer taking that medication combined with the brain damage of spice usage is what, in my opinion, has really changed his speech patterns to be more slow, incoherent, and disjointed.
It may be worth trying to pin down when he stopped taking his ADHD medication (if this is something he wasn't lying about) and when the Spice abuse started and see if there is a prolonged effect on his speech. Obviously the current alcoholism and duster abuse doesn't help. Or the sitting in a closed room full of spray paint fumes after painting a stick he found on the side of the road.