I think I would have to know more before I answered this question. For example, if the claim is that you did not pay a bill, but you have a copy of the check (or credit card receipt), why would you need to pay an attorney to defend against that claim? However, if the issues are more complex, then taking an attorney would be a good idea. While it may be a cost you wish to avoid, it would be better than eternally dealing with the Courts.
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He can always file for custody; however, the better question is whether he would receive it, and that question has too many variables to try and answer in a forum.
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No, that would be unusual. However, you will want to make sure that your motion has been submitted for decision. That may be the problem. If it has been submitted for decision, then I would have your attorney call to find out the status on the motion.
You will need to modify your current decree and if you think the issues are severe enough, you can ask the Court for a Temporary Restraining Order to prevent them from going back.