T.D.
so you, your parents and the child's mother are all in agreement and the girlfriend of the child's dad does not agree? did i get that right?
if that is right then your parents need to sit the child's dad down and tell him how it will be and if he does not like it he can leave and be gone. then you, your parents and the child's mom continue doing whats best for the child. the child's dad needs to tell the girlfriend to stop what she is doing regarding the child or he needs to leave her and be a father first then worry about a gf.
you need to speak to the mother and a lawyer and get things fixed so this child has the best life possible.all parties need to be on the same page and with the blame game going on your best bet is to lawyer up and do it all thru the legal system.