Do your best to try to prove what he did to you was misleading... like that he promised you he would drop the paperwork and he didn't. Make SURE you bring this to the judge's attention, as this is VERY conniving and most judge's will NOT like this AT ALL.
It would be best to bring any type of paperwork with you, to back up your claim that your ex lied to you to get custody. This is serious and i really feel for you. Temporary orders almost always become permanent so you're most likely looking at a situation where you will have every other weekend visitation.
An OUI does not make you a bad parent. Make sure that this is brought to the attention of the court, that you are doing waht it takes to get on the right footing... for example... parenting classes... AA meetings... anything and EVERYTHING.
That, coupled with your insistence that you never would have NOT shown up if you would have known how this worked, and that he lied to you... AND that he is abusive and you have witnesses to attest to the matter.
I would have them write out affidavits, every single person, BUT YOU ALSO MUST HAVE THEM PRESENT to either be Deposed ahead of time - get a DEPOSITION from them if they can't be at court on the actual court date.
If you are at a lower income level, call PINE TREE LEGAL immediately. They can point you in the right direction.