Toxins can stay in your body forever. They are stored in our fat cells so I imagine it's impossible to rid one's body entirley. Yes, you could have flashbacks. If you read past posts you will see that some people have had flashbacks. Last year my father quit smoking cigarettes (no diet change, no other detox) and he experienced 2 days of flashbacks from Crank he had done over 30 years ago. Hopefully all goes well with you and any experience you have on the Master-Cleanse you can enjoy - just be prepared!
As far as BM's, this is my second Master-Cleanse and I am very constipated. The first cleanse I was the exact opposit - the only thing I changed this time was adding the B&P shakes. These will cause constipation if insufficent ammounts of water are consumed. If you're not doing the B&P shakes maybe try another herbal laxative tea. I used Smooth Move and it worked so well that I only drank 1/2 of the tea.