Does a premature closure of a soft sport (or lack of feeling one) automatically mean craniosynostosis?

Not really: I am sure if the baby's head is growing on its growth curve it means the head is able to expand as the brain is growing. If the shape of the head is not irreguler it means there is no real craniosynestosis and the head growth needs to be monitered

No: The actual size of the soft spot can vary from almost undetectable to more than 2 inches across. It can increase in size slightly after birth or never be detectable. The over-riding or ridging of suture lines is a more significant indicator of synostosis, but recognizing that feature takes training.