Probably some combination of the many things you listed, but I think the last (confidence/consistency) is huge for a bit/role player not really counted on to score.
From personal experience, as the end of the bench guy on a really good Class B high school team back in the day who usually got about 1 minute of playing time at the end of the 2nd quarter because 1 of the regulars was sucking air so bad they could hardly walk (we were usually up by double digits by that point), suffice it to say I was scared to death to even get the ball let alone shoot it. I was not a good shooter but whatever mechanics I had practiced went straight out the window on the few shots I hoisted in live action.
I'm not insinuating that Thor is scared, but I think there's always that little bit of hesitancy for guys who get such little playing time.
I don't think Thor's mechanics are all that bad, but playing so little and shooting so few shots over a season make it really difficult to be effective, unless you're just a natural shooter.