The word Apostle strictly refers to the Twelve. More loosely it signifies anyone 'sent out' to spread the Gospel.
I don't know what you are referring to as 500 witnesses to the Resurrection and their wives. No one actually witnessed the Resurrection in the tomb.
The Apostles received news from those who went to the tomb that Our 
Lord was not there. What were they supposed to give back and if they had something to give back were the Apostles somehow negligent in not doing so?
The Greek word ἐπίσκοπος (episkopos) translates as 'overseer' or 'superintendent'. From this Greek word we have the Latin 'episcopus' which over time became 'bishop', 'vescovo', 'eveque', etc in European languages.