Honestly if NK was ever going to irrationality do something Guam is a damn good target.
1. Huge American importance
2. small population
3. NK can still hide behind the threat of SK's destruction.
I don't think so. Even a rational president would not let an attack on Guam (assuming it had any success) meet with a non-response or muted response. An attack on Guam would lead to a major retaliation.