Still improbable as within the Atmosphere and explosion of Ragnarok's Core to which Zero was within point Blank, Zero still wouldn't have raintained intact. Too much heat, too much velocity, turbulance, explosive radious prior, and not being within a Vehicle upon re-entry... And we know Ragnarok never hit the Sea. Thanks to ZX, the remains hit the land, which let Albert obtain and rework the remains.

Zero remains dead.

As much as it pains me to admit, zero is dead. Sure he might have (key word here is might) have survived the explsosion but nothing else. Re-entering the atmosphere would be way to much heat and friction which would have melted him, PERIOD. Even IF he survived that, there's no way he could have survived the impact. Especially because you never know where he will land. He could've fallen on something sharp which in turn would have killed. There is no way he could've survived unless, he was in a vehicle and landed in water or something. And, as stated above water can extremely fatal. Fall off a bridge a hundred feet in the air, DIE, and anything higher than that, DEAD. It sucks that he's dead but o well

I assure you they are all dead. The explosion emitted by Omega was enough to destroy Leviathan, Fefnir and harpuia. Just be cause it doesn't show it, doesn't mean it didn't happen.

Notice their absence in Zero 4... Notice their BioMetal in ZX...

If you just played it, then you'd recall that Leviathan and Fafnir got beaten by Omega. That damage wasn't fixed before the end. Harpuia too was heavily damaged, and was recovering in the Resistance HQ. But only became conscious. His body was still damaged. And when all 3 decide to siege the Final area, they were killed due to being heavily damaged, and within blast radius. The only reason Zero survived was Mother Elf... Whom also vanished, after her power was used up in shielding.

Go pick up the Official Complete Works for MegaMan Zero. It states in there that they died. The info from the developers makes this just as true.