Michael, be careful with Tylenol 3 - they can cause constipation and the longer the poop sits in the colon, the more dried out and hard it will get... Which will not be easy to pass. I suggest that you find a doctor who can help you with a plan - good foods to eat, meds to take... It's lots of new learning. You can also learn a lot from going back and reading old posts on this site... Lots of good information and experience here.
No, Miralax should not cause diahrreah and if it does, you can always adjust the dose (ie. Try a half dose not a full does). Make small changes to your diet... If you add too much fiber too quickly it can really back things up. I take a stool softener and miralax right now with prune juice... It helps to have a consistently soft BM everyday and it has REALLY helped with the healing.
And again, I'm a little worried about some of the things your doctor has told you/not told you. I would really encourage you to seek a second opinion. You will get better... But you need to keep learning and find some good information.