1)A time not for words, but for action <<it is correct  
   A time not for words but action<< why we can't use "noun+ but + noun "  this structure?

2)My objections are, first, that the measure is unjust; second, that it is unconstitutional.<<correct

    My objections are that the injustice of the measure and that it is uncinstitutional.<< Can i use this instead?

    My objections are that the injustice of the measure and it is uncinstitutional.<< Can i use this instead?if cannot ,why?

3) In spring, in summer, or in winter <<why need comma before the word "or"? 

in spring, summer or  winter<< but this time ,we doesn't need to use a comma to seperate "summer" and "or" ?

Thank you^^