And some things about magic in general (this is mainly CT concerned, of CC I haven't given it too much thought).
Shadow Magic seems to be drawn along certain mythological lines. For instance, in many cosmologies, some kind of Darkness existed before the universe. A kind of Chaos or Void. Now some (say the Judeo-Christian worldview) say that Light intervened and created the world. Others say that things simply emerged from the Darkness.
Thus, the other magical elements emerge from Shadow. And of course, they all must balance in their way. Spekkio specified that.
The one Shadow user we know of for sure is capable of a great deal, including the use of other elements, reinforcing the idea that Shadow is a sort of Original Element. The idea that he somehow can use elements outside of his Innate is preposterous in my opinion, without some sort of aid (like Lavos or even the Dragonian Elements). I mean, if he could do it, what stops Crono from using Water magic? All creatures in the game that ever use Shadow attacks always seem capable of using the other three elements (Spekkio, Lavos; a more complete list would neccessarily require analysis of all the games spells). Characters who can't use Shadow, never seem capable of using spells outside of their respective Innate (Crono doesn't use Water spells because he manipulates the Lightning/Heaven element).
One's "innate" element almost seems like a frequency one is tuned too. Crono can pick up on the Heaven/Lightning Element, is tuned to that, and can manipulate it because of that. He can't "change his tune" so to speak, as that would require him to suddenly become a different person. For people who are "tuned" to the Shadow Element, using others is a matter of simply dividing up the aspects of the Shadow. Being able to "tune in" to the underlying Shadow in all magic and the world at large (rather than just one aspect of it, i.e. Lightning).
If you accept that, without some kind of outside help (I''ll say CC Elements because they are better examples), you can't use magic outside of your own Innate Element, then the fact that Magus can use Lightning, Fire, and Water shows that Shadow is at least related to the other elements, in a way more fundamental than the others are related to each other. It definitely reinforces that Shadow is a kind of original or "primordial" element.
Some manipulation of spacetime is at least IMPLIED by the game, in the fact that, Magus had to somehow tick Lavos off enough to leave the Pocket Dimension, or he had to somehow get to it himself, from his castle. Whether or not Shadow allows time travel/gate manipulation is still up in the air (he does appear in Radical Dreamers; regardless of whether that is in the Main Timeline, he had to have gotten there SOMEHOW, not that magic is the only possible way).
What I can't explain is how Shadow damage can be caused by "Antipode" if Lightning is also a part of Shadow. Could be a gaming element, without relevance to the story. Could be what someone brought up before, that Heaven/Lightning is somehow a dualistic opposition to Shadow, but then how does Delta Storm then do Shadow damage if Lightning is also a part of it? It would be interesting to experiment with this. The Magus fight (and the Lavos projection of Magus) are probably the handiest ways of seeing which attack does what sort of damage.
Tentatively, I suggest that the combined Water and Fire elements present enough of a mix in order to break through the barrier, but aren't in fact the true Shadow element. The combination creates a sort of Shadow effect. Why? Probably has something to do with how the elements add up exactly. A mix of Fire and Water is closer to being Shadow, than just Fire or Water on their own. The barrier that was made might not have anticipated this sort of mixing and thus couldn't defend against it. Its hard to puzzle out this point, really.
Enough incoherent babble for one night, I think.
