My son has had terrible chronic constipation and the only thing that has helped has been Miralax. He has been taking it now for about 2 years. I am currently in the process of weaning him very slowly off of it because he is now old enough that we can cope with some of the behavioral issues relating to the constipation. It has taken us a long time to get there. I've tried getting off the Miralax 2x in the past but couldn't get my son to do the behaviroal stuff until recently. Lately we've been really successful
with good poops 2x a day!
Anyhow - We have had no problems with Miralax. No cramping, no complaints, etc. A determined non-pooper can hold in even the softest poop for a suprising number of days (7? 10?). The thing that Miralax does is helps draw water into the bowel so that the poop stays soft. You may find that a very large relatively firm poop is passed quickly followed (e.g. 5 - 30 minutes later) by a large softer, followed by a large soft, followed by very soft, etc. You may be seeing the same thing with the enema. This is because no matter what, the longer the poop is in the bowel it will become more firm. I don't think the Miralax caused diareah - I think not pooping and passing 5-7 days worth of poop led to some soft stools and blow-puts. Mostly with Miralax my son had soft easy to pass stools on a every-other-day basis.
The problem with chronic constipation is that it is quite painful to pass the hard stools, and also quite painful to hold them. An impasse, so to speak. Using Miralax (or whatever works for you) can help ease the pain of passing the poop and break the cycle. In my reading I think that it is recommended that whatever works (medication or whatever) be used for several months to break the cycle. Then the behavioral things and diet need to be very consistent after that.
Magnesium, BTW, can cause cramping - I'd be careful with it, IMHO. It is very effective in the short term but I don't think it is a good idea for long term use. (E.g. it is used by GI doctors to "clean out" the gut before a colonoscopy)