I sympathize with your predicament. I have an 8 year old daughter. She can be quite a handfull even without these other issues. My daughter would also freak if we tried to give her an enema. I would have advised that she assist in giving her mother an enema first. She would have understood how things worked and that the discomfort is minimal. That may have gone a long way in relieving the anxiety that I am sure she felt.
I assume that stool samples have been taken are no parasites found. Doctors consider these cleansing products to be quackery, although not usually harmful to healthy adults. A childs tolerances are different. For example, during an enema or colonic, the body absorbs water from the colon. If too much water is absorbed, the bodys electrolytes can be pushed out of balance. This can be a problem especially with young children.
While I understand the father's frustration, a whipping is not likely to help.
The Supernanny show does a pretty good job of demonstrating some strategies that work with difficult children. Begin with a clear list of rules and expectations. Stick to a routine. Be consistent.
I just had another thought. Some people subscribe to the thought that we train ourselves out of the routine elimination of wastes. A baby will usually have a bowel movement within an hour or so of being fed. After each meal, have Meg spend 5 or 10 minutes or so on the toilet. Not straining. Just encouraging her digestive track. A little abdominal massage as well. Speak with the school nurse so that an accomodation for this can be done at school.