Yeah that's insane. First sign of blood in poo should have scored you an exam. You probably have grounds for a law suit if you are that sort of person. Thank the powers that be that it was benign, though. You could be telling a much different story right now!
Anyway, I wish you luck with your reversal. Your fears are all valid, but if she saved part of your rectum along with most of your colon, then you will most likely be in much better shape and have an easier reversal than many of us here who have had our entire colons and rectums removed. Even those with no colon or rectum at all are usually able to control bowels after the reversal with just the anus and sphincter muscles. (Haven't had my reversal yet - just know this from extensive reading and online chatting.) And even those with leakage problems in the beginning are 99% resolved within the first few months. So keep your chin up... you'll do great.