Would a regular abdominal/pelvis CT scan with contrast and barium suspension reveal moderate to severe ulcerative colitis / Crohn's?

Most likely. there would be visible changes on the CT scan unless the colitis is very mild. With severe colitis, the CT will invariably be abnormal.
Much of the time. All structures from diaphragm to groin will be visualized by a contrast ct scan. Consuming oral contrast distinguishes gut from surrounding structures; IV contrast better delineates solid organs. Even blood flow, neurologic impingement, hernias, etc. Are made apparent by ct, typically to a resolution of about 5mm.