The abilities available only to the solo rock-on users are:
May I point that comparing boss form attacks to player form attacks rarely gets anywhere; see X5. Since A-Trans is a direct copy, there's no reason it should be lacking attacks. It doesn't lack any of Model ZX's (although Aile's Fission doesn't toss up as many rocks, but that and the consumption of biometal energy are the only differences). But when you limit a boss's attacks to those that are designed for a player's gameplay balance, it makes a pattern-based CPU ridiculously easy to fight. And Model ZX is the perfect example of that.
Why would adding X's DNA to the equation suddenly make them look more like themselves? Their own DNA data is enough for them to appear like themselves. X's DNA would make them look more like X, not the other way around. Even worse for Zero, who has a copy of his original body, the very definition of a copy doesn't allow such huge changes as suddenly adding in X's DNA to the mix. All aspects of X that are in Zero are those he had from his very creation by Wily, and the upgrade of the Z-saber.
Really, the appearance of the models is all thanks to the designer and the game making process. Because they already made the double rock on designs so close to the original, they had to deviate for ZXA. Because they already made ZX so close to the original, they had to deviate for Z.
What statement do we have to say that Inticreates designed Model ZX before they did Model Z?
Although this is PURELY speculation, I'd always attributed that to Model X enabling otherwise dormant data within the other Biometal. The ZX Buster is what influenced my thinking there. The pistol is an obvious reference to Zero, since X never used one, but Model Z doesn't have it.
Model Zx lacking O.I.S is way powerful enough that it can handle & destroy almost everything. Minus the Air & Water mobility it's still a very Destructive Model...than Ox.
Almost, but not quite. Even when you ignore OIS, Model ZX's only upper hand is the fact that the max charge buster is a 3-shot spiral, which annihilates stage enemies but makes no difference against bosses. Model OX, on the other hand, has a more powerful low-charge buster, longer saber range, and a far more powerful charged saber (OX's is 16; ZX's is 12).
The Dark Elf was being used to control the Reploids. Since X had become a Cyber-Elf himself, he was out of the Dark Elf's reach, not to mention that he had the ability to suppress the Dark Elf's power. Zero's immunity to the Dark Elf is a result of the latter being created from samples of the Sigma Virus, which doesn't affect Zero normally. This also serves as indication that Zero's copy body has similar, if not the same properties from the original.
As I recall, Telos indicates that the Big Four are immune to the "broad" control (despite Harpuia succumbing to the Baby Elves directly in Z2), so one would think that X is as well. Most do not believe the Sigma Virus affects him normally, either.
Personally, I think there was a reason Zero mentioned himself and X during Z3's dialogue the way he did. The vibe I got was that they were trying to show how X is special along the same lines as Zero, but since he happens to be disembodied, it wasn't really applicable to him anyway.
May I also say that I think you hit the nail on the head with Model ZX and gameplay earlier. I maintain that Model OX is really what ZX should be if not for game balance. And seeings how their differences are mostly separated by OIS, I really don't see what would be wrong with a late-game upgrade.
Although, to be honest, if Inticreates hopes to maintain game balance while returning to Model X as the starter (which is what we SHOULD be seeing, looking at ZXA's ending), then I think they need to re-think their damage system altogether. They have a much stronger benefit to charged shots over rapid-fire than X-series games did, and that's why Model X's double-charge is so delightfully broken. I'd rather see them change the formula than nerf the abilities.
Ignoring the fact that Model P has no elemental attributes, I personally believe that OIS is an effect from Model X boosting the power of the originals, since it's already been established in ZX that a Double Megamerge draws greater power. The OIS is so far the best way to convey that from a gameplay standpoint. And as you said yourself, Model O and Model W are special cases.
That and generally shorter charge times and greater attack variety. Besides Model F none of the Big Four metals have a low-level charge attack when not paired with X.
ZX3. model ZX. Giga attack or bust.
Quoted for absolute truth.