the thing that was actually buggin me is...i was just re reading infinite crisis 7.. and in that issue when superboy prime flies towards oa...martian manhunter says something in the lines of " anyone and everyone who can fly, take to the skies"... so almost everyone who can fly follows prime into space.. but later it is shown that wonder woman had stayed behind...along with batman etc.... so..why didnt diana go with the other flyers??

I think she needs a breathing apparatus to opperate in space for any length of time. Could be wrong though.

Not sure if there is a consistent application of this (which is true of most heroes) but I think generally yes she does need one.  

She's been all over the place, either needing a suit or being just fine with a small oxygen mask and in some cases even being able to hold her breath long enough to step out into the ebon sea in the sky. Personally I think she could hold for a few minutes before she starts to feel the effects, with prep I like to think she could last about 15 minutes without external aid before she gets major damage like lungs ripping apart, arteries blowing up, blood boiling etc ...

As for flying, I don't see why not as it's magical flight.

its not consistent for any hero tbh. I've seen Superman wearing a suit with an oxygen tank and ive seen versions of him flying into space like its nothing(like in Man of Steel). It's just another example of how loose comic book characters are, they have a general idea of what the character is meant to be but besides that writers do whatever they want for the story