The danger of air embolism is pretty remote. It requires gas get into the circulatory system in fairly significant quantieis, and that is improbably with either douche or enema.
Carbonated solutions tend to be very hard to hold, since they tend to release the dissolved carbon dioxide on contact with the bowel wall. They generate a lot of gas volume very quickly.
The sugar would be a feast for the colonic flora, so that would generate even more Carbon Dioxide if you were able to retain it for any length of time.
The Coca-Cola Douche is actually a moderately effective birth control when done prompty. It is both cleansing, and the unfavorable pH and osmality are very hard on sperm.