I agree!! Sadly, I think a whole lot more happened between where Daniel was picked up & the house. I also think when Cowan was "confessing" to Arnold, he may have been a bit wary & was weighing up how far he could or should go with all he told him. He needed to stay in the good books with Arnold. To tell him there was another involved in the abduction, in his mind, may have led to problems between himself & Arnold & he perhaps didn't want to risk that happening.
In my opinion, self preservation is what it's all about with these creeps.
I think you're absolutely correct here Marley.
Another person, regardless of their level of involvement, still means a witness .... And as Cowan was believing this was a super strong crime "family" the stupidity of having someone else be able to place him with extreme detail there might not have been something he wanted to admit to.
One statement that strikes me "I didn't even get to molest him or nothing"........ Like his biggest regret was missing out. But it's sort of a weird remark to make, out of context with the rest of the confession - it just sort of pops at me. Like "I didn't even get my turn" (but still had to dump the body) - kind of petulant maybe? I'm not sure. But it does seem to fit better under the assumption he wasn't alone.
I'll leave it to the psychologists and linguistic analysts - but that statement to me feels like there is a great deal unsaid.