Is naming a newborn after an uncle who died in an accident, and then naming another [child] with the same name who dies in the womb, considered a forbidden act of superstition if the family insists on not naming [any child] with it again?

Fearing a specific name or expecting evil for a newborn because of it falls under blameworthy pessimism. The death of a person who bore the name does not affect the life of the newborn. The father has the primary right to name his child, and if he insists on naming his son after his paternal uncle, there is no harm in it, and he should not be opposed merely due to pessimism about the name.