Will a video colonoscopy completely rule out crohans disease, ulcerative colitis and other ibd. will it also rule out cancer?

Not all: Colonoscopy will rule out colon cancer , ulcerative colitis , ( early uc in remission may not show the disease ) but not crohn's involving only small bowel only , other ibd some times may not have visible pathology . All of these diseases need biopsy as a proof.

No: Crohn's disease can still be in the small bowel and not seen by colonoscopy. It can rule out colon cancer and ulcerative colitis.