Sorry I have no advice as I am yet to have my second, but I am very interested to hear ppls responses as I had a retained placenta with DD, and like you it was more painful than the birth (I eventually had to have a manual extraction under GA in theatre) - the MW said to me when I was screaming in agony whilst her whole arm was inside of me "this would have been much easier if you had had an epidural!", oh gee wizz excuse me for having a natural birth! So I am really hoping it doesn't happen again with bubby no. 2.
I too had a MROP with my first, after a bit of feeling of my stomach and the injection i was taken to theatre to have it removed. I didnt have the same issue with either my second or thrid labours. I had a fantastic dr with number three who talked me through a lot of things and encouraged me to have the needle for the placenta when the baby's head first comes out (before her body was even out) that way i still was having contractions and he said it made it easier for my body to get it out naturally. I took his advice and the placenta came out quickly with no hassles at all.