Yes. You must terminate your parental rights in order for a legal adoption to take place. All your obligations and rights to the child are terminated at that time.
No. If a child is disabled prior to the age of majority, most states have different child support guidelines. Check your state laws. In many cases it can actually continue past the age of majority.
The persons in custody of the child must file for legal guardianship and request the court modify the child support order. The person needs legal standing to receive child support.