Overall I agree, but colonoscopies will catch more and also potentially find other problems that aren't cancerous, and aren't related at all that they can provide treatment for.

Prep should not affect your long term bowel movements, and while I won't convince you that your perception of the cause is untrue, it's essentially just a liquid fast with laxatives -- it shouldn't cause a problem.

Possible that something was damaged on the way in or out though.

As for your last comment: all the yes. Anaesthesia is some terrifying shit.