Is it possible for someone to have a mild cluster headache? If so how likely? Can it vary in severity throughout a cluster period? If not? What can mimic this disorder?

Mild Cluster. Headache may be hemicrania continua if daily occurs only on one side without consistent features of Cluster) There is no tearing, nasal discharge or flushing with hemicrania unless it is a paroxysmal intensification. Hemicrania almost always responds to indomethacin (25 mg three times a day with food)
Cluster headache. Cluster headache tend to be very painful. They can be difficult to deal with. There can be different degrees of pain but most are intense. Check with your doctor if you're having these headaches.
Cluster Headache is. A label for a pattern of vascular (i.e. migraine) head pain activity. That does not mean that the pattern (s) have any great diagnostic, useful significance, though promoted by some to imply they have some particular expertise in understanding or being effective in knowing how to help people stop such patterns. Buyer beware.