I am not sure how it works in Ohio, but in my jurisdiction, once a child turns 18, unless they are still in "high school" or have a permanent disability requiring ongoing care, the support obligation ends. However, check with a local attorney to see if your State is different.
I would suggest contacting an attorney in your jurisdiction. However, if you file for support, the court could possibly take into consideration income received from the settlement in calculating past and future support. However, if he has not fought for custody or parenting time rights to date, he may seek determination of parenting time in hopes of lowering his support obligation.
You should contact a family law attorney in the state of Indiana and file for enforcement against mother. Being that Indiana is where mother resides, they will have jurisdiction under applicable state statutes to enforce and enter an award against the mother for arrears even if the child is over 18.
I am not a texas attorney, so I would of course recommend that you seek further advice from a lawyer in your jurisdiction. However, generally you can petition the Court of proper jurisdiction for genetic testing to determine whether or not you are the actual father. If it is then determined that you are not the child support garnishment should then be ordered stopped.
I am not a practicing lawyer in your jurisdiction. However, the genral answer is no. QDRO preparation is a very specific area of the law. Therefore, I suggest that you consult with a QDRO lawyer in your jurisdiction for more information.
Since there are no custody agreements and/or court orders regarding custody, both parents are entitled to access to the child. If the child is being withheld from you, you should consult with a local attorney to see what types of emergency relief are available to you. Again, if you cannot find him for service, speak with a local attorney and inquire about service by publication or alternative process granted by the court.
If there is no custody agreement or court order stating otherwise, either parent can take the children. However, check with a local attorney as generally there is emergency relief available in this type of situation.