決して~ない => never; by no means ~ N3
[決「けっ」]して~ない => Won’t, isn’t N3
Will, Decision, Doing
でも[差「さ」]し[支「つか」]えない => It's OK if A, it doesn't matter if A N1
Need/Requirements
に決まっている => certainly; I’m sure/certain that; it must be the case that ~ N2
に[決「き」]まっている => A is a given, A must be the case N2
Information, Report