I feel like your misunderstanding what effect the OD actually has on kaiju( OK maybe it's harsh to expect everyone to know cus even the humans in the story misunderstood the biological principles of Kaiju) I'll start with Ghidorah. he as is explained never needed oxygen, what's never explained is why. simply put he substitutes breathing air for something similar to what earth kaiju can do to supplement their bodies energy needs, he absorbs radiation, why dose atomic radiation do so little for KG? because he's charging off a different kind of rad, same ones our atmosphere only KIND OF stops-cosmic background radiation. so as implied before a Kaiju's body is only able to properly power it's self because of rads and on earth an EXTRA effective form of radiation is on the proverbial dinnerplate thanks to humanity the aforementioned atomic radiation. Gojira's kind in particular has and IMPRESSIVE adaptation that allows them to do this BETTER then other earth born kaiju to the point that they may in fact be able to pull the same trick of substituting breathing for a sort of radio-synthetic respiration. what proof is there to collaborate this? cus it wasn't the fact that the hit was indirect that mattered in the movie as the weapon( OD ) is fundamentally a radial disruptor for the sub-atomic bonds of oxygen atoms, the explosion is simply a trigger for this effect. in short the OD wasn't effective in the movie because it never actually deprived KG and Goji of their ability to respirate( well ok it DID do that to goji but he had a back up) and likely even for standard kaiju it might amount to a powerful sedative at best (and if humanity gets TOO slap happy with OD's in the future-the reason future generations of earth Kaiju may stop needing air) in short? I doubt goji is gone BUT if the humans and KG thought he was dead he may be out of the game for the foreseeable future cus his body may never have gotten the atom bomb refresher he badly needed to continue the fight and KG would have in his arrogance failed to double tap goji just to be safe, in short Goji is down for the count for like I'd say about 10,000-100,000 years ( there is NO data on exactly how much a kaiju's body is deprived of energy by the OD and Goji's abilities throw an extra wrench in the works for pin pointing ANY viable range other then "lifespan of an empire" to "Kaiju hibernation before humans started nucking things").

sorry for the GREAT WALL OF TEXT but I'm a big frigging nerd, also please don't look at this as an attempt to ruin your AU ( I love what you've done with ammy ) I just like to provide comprehensive lore.

did I mention I'm sorry for the text wall?