In Gundam SEED cloning hasn't gotten to the point where they have rapid growth techniques in cloning. Which also lends itself against the Neo is a clone of Mwu theory. Since you'd think Mwu would of found out about his look alike. They are practically the exact same age.

Anyways in the SEED universe its possible to survive in space without a normal suit for a limited amount of time. Lowe did it himself in the Astray Manga. What really happens in space is known as decompression. Space is a vacuum obviously and if you were to exit into space without protection you would be affected by the shifting of pressure. Basically everything inside of is gonna be pulled on. In Mwu's case with that helmet floating he would have no protection. He would likely start to bleed from the ears and nose and possibly eyes while holding his breath. This wouldn't kill him instantly thou. If the cockpit had been filled with air it would of sucked him out once it was exposed to space via opening the hatch if he wasn't latched in.

Pervert kid, "mwu died he didnt surive end of story". I don't want to spoil your fantasy but while that view isn't the majority view it really wouldn't matter. The way things are going Bandai/Sunrise wanted Mwu back and that seems to be what they are doing. Stacking the evidence in favor of Mwu being the original with a good old memory rehaul. Luckily thou it appears not to be a permanent or atleast not a perfect procedure as some fragments of memories can reemerge. Stellar's case for example with her fragmented memories of Shinn.

The only real evidence that puts a real "how'd they do that" to this spin is that we see the cracked purple helmet floating in space. A scene that later was ordered removed from the trilogy compilation for reasons unknown. Last time I heard the later releases took precendence of works. Perhaps similarily to how Zeta's compilation is the Story as Tomino views it should be instead. But he's Tomino. He can change the timeline anyways he wants. Perhaps thou the situation is different with SEED. But when they do something like that in a compilation it almost always has some relevance.

Sadly they cant remove the helmet scene from the R1 dvds since they are supposed to be unedited from the japanese showing. Gotta hit the sac now, work at 7:30.