Mild elevated bilirubin
does lead to kernicterus
.Kernicterus leads to brain damage
and sine your son is grown normally physically and mentally he never had kernicterus.the vein you are talking about must just normal variant or probably is some hemangioma
which is a collection of vein under the skin hence becomes more prominent when heavy exercise or high temp.you might need to discuss it with your Docter about it so he can examine it.
Middle child of your being on the darker side is also not due to phototherapy
probably is familial. The test required if a child suffers from kernicterus is clinically assessed with high pitch cry,convulsions at the neonatal period ,MRI brain can be done to find the the brain damage.