My blue still holds true, they just had to break down the shed door earlier that day, probably when they were picking the stages for the murders.
Repeat this: “Vin’s head was brought inside the garage after Vin was dead”.
Plus: When the group left the Garage, they saw Vin’s head. that doesn’t mean it happened immediately after, I can say Kalladin slipped right after “the group” wasn’t inside the garage anymore, placed the head there and ran away unnoticed. Continuing with the Arya accomplice in my theory, as long as Arya leaves the garage, the people inside it don’t constitute “the group” anymore, so Kalladin has leeway to escape unnoticed, and even if he’s seen by Arya, being an accomplice, she won’t say anything.