Can an abdominal CT to rule out kidney stones and other abdominal pain (w/o)contrast detect colon cancer?

Doubtful. Unenhanced cat scanning of the abdomen is not reliable for diagnosing colon cancer.
Yes. It sometimes can but it is not able to do so reliably. The cancer has to be relatively large to be seen. Special ct colonography is performed at many centers for screening purposes. My preference for colon cancer screening is colonoscopy as biopsy can be performed of any suspicious areas during the exam (not possible with ct colonography).
Sometimes. Secondary characteristics, mass, obstruction may show but colonoscopy looking as entire colonic surface has a better chance for early -curable lesions!
No comparison. Ct ; MRI are indirect visualization modalities. Unlike colonoscopy (a direct optical exam of the colon lining), small lesions may be below the detection threshold of scanning. Also, colonoscopy affords the opporunity to biopsy the colon lining, to remove upon detection colon polyps (finding them on ct or MRI will only prompt undergoing a colonoscopy to get them out), as well as therapeutics.