A court cannot retroactively alter a child support obligation. Even if the court did not have the right to make a ruling that applied to the November 2009 child support when the Order to Show Cause was filed in December, it is not the first time and will not be the last time that a court made an improper ruling.
Again, the court could order a "waiver" of child support as you call it for any period of time following the filing of the current OSC and prior to the court date. However, the Court previously stated that it would not "waive" that obligation. I am not sure that the elective surgery aspect is of any importance, since the father was already homeless and not working.
However, if the Court "waived" support retroactively before, who knows what the Court will do this time around.
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The Court can retroactively modify support back to the date the father filed his motion to stop child support. I know that probably puts you in a financial hardship and I do sympathize with your predicament.
That being said, there are definitely things you can do to ease your mind and help you get better results. You might want to consider filing a motion with the Court asking for a seek work order--this requires the father to do things such as to apply to a certain number of jobs every week and keep a job log (and provide the records to you). You can also ask the Court to impute (attribute) income to him--meaning that you are able to prove that the father has both the ability and opportunity to earn $X. Having an attorney will definitely help you out in this area.