You mean there was no differentiation between the 12 and the 72?  That's simply not true.  Whether the word "apostle" was interchangeable with "disciple" as a matter of language is irrelevant if, in fact, the two groups were distinguishable.  And, of course, they were.   You'd have to deny that there were the 12 in order to say otherwise.  I don't think you're willing to do that to win an argument about an all male priesthood. 

So the contention that the words were interchangeable may or may not be true,  but it certainly can't be used to alter the fact that there were the 12, and that by whatever name, Jesus restricted this inner circle to men.  The question is:  In view of his nature, what is the most reasonable explanation for his having done that?