I shared my story of renouncing here:
https://forumserver.twoplustwo.com/3...65/?highlight=
To answer your question about the actual certificate - it came several months after the actual day I went to the embassy to do it. Basically you go through the process and it gets submitted to the US state dept. Once they approve it you get the certificate - you definitely don't get it the day of and by the time you get it, the process is completed, no backing out.