You seriously need a good criminal/DUI defense attorney. From the few facts you have presented, you will have a difficult time without legal assistance, and even with legal assistance, while the damages might be lessened, you will not get off scott free without a miracle, and one miracle is the state's inability to prove its case. Speak to several lawyers and hire the one you like the best.
If your income has decreased, you need to file a motion to modify child support. The sooner you do it the better as your support will be modified effective the date you serve notice of your motion on you ex. While you can try to accomplish this yourself, you are much better of hiring an attorney to help you. You are fortunate that here are many good lawyers in your area. Speak to one o more and hire the one you like best.
Your ex wanting to "prove up" does not make sense. Speak to your...
Look around your mother's home for paperwork. Examine tax returns. Go to any safe deposit box. Check with banks where you know your mother banked. Speak to any known attorney or accountant your mother may have hired. Without organized records, you are on a bit of a fishing expedition. Certain investigation services will be able to locate investments for a fee;.