Is passing additional parameters through function pointer legal/defined in C? [duplicate]

Answer 1

The function call is undefined behavior.

(C99, 6.3.2.3p8) "[...] If a converted pointer is used to call a function whose type is not compatible with the pointed-to type, the behavior is undefined."

For information note that a function type:

(C99, 6.2.5p20) "[...] describes a function with specified return type. A function type is characterized by its return type and the number and types of its parameters."