There is really nothing you can do their not minors their becoming adults in a year. They gave a choice who they could go with since their not minors
Custodial interference. see links
Generally, only under circumstances in which there is clear evidence that the child is in an unsafe environment, and has been reported as such. And in that case the parent must file for an emergency order with the court of jurisdiction. It will be followed up by a full hearing within a couple of weeks.
It depends on the state you live in. Some states allow the custodial parent to opt out of child support. Other states don't give you that option, and will collect the child support even if the custodial parent refuses to accept it.
Not even if he wasn't in arrears.
only with the court's permission
Either to the custodial parent as the obligee, or to the State as reimbursement for public assistance.
There would be no desertion or abandonment since the parties are divorced.
Karma is a great thing. The non-custodial parent will eventually lose if they did not file expressly to avoid the refund going to you. However, look at the IRS site for exceptions which allow not filing.
no
It seems unlikely that a non-custodial relative would have "standing" to request a change in child support.
Usually, no. The non-custodial parent is expected to make at least a minimal contribution - in Illinois, 20% of net income for one child - regardless of the custodial parent's income.
In some states, if the custodial parent moves more than 65 miles "as the crow flies" from the original address at the time of the custody agreement and does not get written permission to do so from the non-custodial parent, the court can (and sometimes will) remove the child and place him/her with the non-custodial parent. At that time the non-custodial parent will be given full custody of the child and even if the first parent moves back, they probably will not regain custody again.
If the custodial parent agreed to deviate from the original child support amount, she can increase it back to the original amount if there is a court order. In additional, the non-custodial parent may have to pay back child support for the time when he paid less.