You can file a modification of child support but unless the economic circumstances have changed significantly you may not get the results you want. You can ask the Court to reconsider it's judgment if you can show something that was overlooked or miscalculated.
If you share joint custody and wish to become the sole custodian, you could file a complaint for modification of custody. If he just has visitation and you'd like to attempt to decrease it based on your girls' wishes, you could instead file a complaint for modification of visitation. Either way you'll want to sit down with an attorney about it as they are certainly not the types of actions you'd want to try to file alone.
They may make an Election Affidavit at age 14 but the Judge does not have to follow it (although they often will)
At 16 they can freely choose where they go without the need for the Court to approve the move.
It would depend on what crime he was convicted of. Also, if you want to challenge the sentence, there are specific time limits you need to abide by, so make sure and speak to a criminal defense attorney as soon as possible.
Georgia law states:
"In all custody cases in which the child has reached the age of 14 years, the child shall have the right to select the parent with whom he or she desires to live. The child's selection for purposes of custody shall be presumptive unless the parent so selected is determined not to be in the best interests of the child."
So essentially, yes, the 14 year old can choose to live with whichever parent he or she chooses, and the Court will abide by this unless evidence shows...
You can file for permanent guardianship of your niece, but you will have to attempt to locate her mother to serve her with the papers. If you cannot find her, you may have other options available. It would be best for you to speak to an attorney to guide you.
Cobb County has specific rules that you must follow before you can be appointed a public defender. I would recommend calling them as soon as possible at (770) 528-1950 to begin the process of having an attorney appointed for you.