So do you pester authors on other forums too about offering their opinions?  Why have you chosen to single us out?  There are books available from authors which are not attorneys.  These books give tips on how to be successful in the court room.  Have you written them?
Specifically with regards to an error by the court, the court is at fault, and therefore the judgement would be retroactive back to the original date of the order, not when he caught the error.  I believe we call that the statute of limitations.  If the court ruled in error and cited no reason for the erred judgement, it can be appealed, period.
When I state my opinions, I consider how things really work in the court room and what the impacts will be.  It is illegal to lie in court, right?  Attorneys and parents do it many times every single day in court.  Neither are ever held in contempt for such an infraction even when they are caught red handed.  The court is not going to hold a party in contempt for asking to make an order retroactive.  There is no harm in asking.  Many times with child support, you don't even deal with a judge, you deal with a hearing officer, who is not a judge.  He may or may not know the actual law surrounding the request.  The party making such motion would be well-advised to know the law.  Many attorneys need to look up the laws themselves to make sure.  Why can no one else do that?  They can!
I will agree that for most people, hiring an attorney is a good option.  Legal work is serious business and should only be attempted by those willing to spend the immense effort and time required to learn the laws of the land.  However, if you think real hard about it, why would someone be reading and posting on forums if all they wanted to do was call an attorney?  Shouldn't the question be, "who is a good attorney?"  Then this website would only be a list of attorneys which can be found in the phone book.  Case and point, people want information, opinions, stories, case law, and moral support.  That comes free here.
I will add this - attorneys are salespersons just like all others.  Some are honest, others are crooked.  I have heard many times of an attorney promising the world in the free consultation, but once the retainer comes in and the bills start stacking up, the truth comes out that the outcome will be drastically different.  This is when most parties decide to go prose because they are tired of getting the run around.  Usually successful prose parties are of the same intellectual ability as an attorney, if not more so.  An attorney once told me "a pissed off smart attorney on the other side just makes me laugh, a pissed off smart prose on the other side is dangerous."  He said the reason for this is because a prose can actually make more work for him because a prose will spend all of their time on the case, where he cannot.  Nothing angers an attorney more than being beat by a prose.  So hiring an attorney or going prose are both a gamble in the family law court room.  Everyone has to make their own choice.