Is the guilty man actually the one who planted the left stalk.

Yes

Did the thief alter a stalk, or send somebody to alter it for them?

the thief did alter one of the stalks

Did the guilty man either push his own stalk one inch farther into the ground, or raise the other stalk one inch out of the ground?

I have more of these scenarios, of varying levels of difficulty, if this is a game you guys enjoyed playing.

The thief snuck down to where the stalks had been planted in the middle of the night to attempt to remove suspicion from himself by cutting one inch off of his stalk. In the morning, his stalk was still one inch shorter, and the magician revealed that his magic had been unnecessary, since he know the culprit would attempt to vandalize the bamboo. The guilty man immediately admits to his crime.