Magbesium citrate is ok for the flush but you will need to take 3 heaping TBSPs to get it to work similar to epsom. There are no major risks to doing a Coffee-Enema and it does not process in the body like drinking coffee. Very little caffiene is absorbed when doing a Coffee-Enema so no worries.
A high enema with water and Sea Salt should do a pretty good job of cleaning the colon. You can also drink saltwater and it will cleanse from the top down.