I agree with your overall cautionary words (and completely agree with stool softeners as preventatives then Miralax as second-line defense), but I think it's worth mentioning that you *can* manually treat the problem yourself at home, even when the constipation has been too prolonged and the feces has gotten too impacted to pass. I agree that a month without a bowel movement is probably worth getting treated professionally. However in cases that are less extreme but still severe, there is another option besides OTC laxatives on the one hand and an ER visit on the other hand.
If the mass itself has already gotten so large/hard that it can't physically be passed in a safe way, I'm not sure that even pre-surgical laxatives by themselves would be able to soften it sufficiently, though they could help by providing extra lubrication (which is why glycerin suppositories are a good OTC option). Once the impacted feces has reached a critical mass that can't be passed safely, it seems necessary to physically break it up before removing it. And for the price of some rubber gloves and mineral oil, you can do yourself a lot of good on that front -- just using your own "digits" to chip away, little by little.