TWDuke you’ve got the right of it. The Oxygen Destroyer basically skeletonized the big G (along with its creator) in Tokyo bay. I don’t recall there ever being an explanation for his ressurection in the original movies, they just brought him back.
In the 85 reimaging, complete once again with Raymond Burr inserted into the American release of the film, they mention that, way back when they never found a body.
(The skeleton seen by the audience is, however not retconned out of existence, it’s just not mentioned.)
In the third (I think) 90s flick, the new Godzilla vs King Ghidorah, they time travel right around his destruction by having his transformation from semi-benign dinosaur into Godzilla not take place until the year 200x, as quietman mentioned. “Causality be damned, we need more Kaiju Fightin’ Action!”
And in the 2000 rebirth, the whole thing gets pretty much ignored in favor of more neat crush, crumble and chomp type goodness.
And as to the '04 release of a new MechaGodzilla flick, you have a link to any info on that, mate? It’s possible that the film in question is actually the 90s Mecha-G movie, which seems to have never gotten any major distribution in western markets. All the rest, G-85, G vs Biollante, G vs KingGhid, G and Mothra, G vs SpaceGodzilla (yeah, really; fun movie, actually) and the final G vs Destroyah were relased to video, or American movie channels (AMC and Showtime, at the least) but I’ve never run across the “new” G vs MechaG m’self.
The Skeezling is three. We happened across G vs KingGhid on Showtime a couple months back, and a new fanatic was born. So I’ve been immersed in Kaiju all over again the past while. And they’re just as much goofy fun now as when I was a wee lad. 