Personally, I agree with what Eske said a couple of pages ago about the identity of Eclipse Magus. I can't quote it because I don't retrieve the original post xD
In a schematic view:
1) Crono and his happy crew (including* a Magus that we'll call "Party Magus") go and defeat Lavos. The player gets the happy ending, Crono marries Marle, Lucca discovers Kid, Frog gets his human form back, etc etc etc, and Party Magus goes back to 12000 BC in order to search for Schala. The player unlocks the Dimensional Vortexes.
*note: we don't really have to think about nothing if we don't have saved Magus on North Peak. The canon says that Magus is alive and well at the end of Chrono Trigger, and that he undoubtfully joined Crono's party. Since here we're discussing the canon, we can safely assume that Magus IS in the party.
Let's think about it. Why do these Vortexes open? We only know that the DVs are, somehow, connected with the Time's Eclipse (or the Darkness Beyond Time, as you wish to call it). Maybe the three clones REALLY are the Ghost Children, or something like that. But let's stick to some known facts: when the DVs open, SURELY Lavos is already in the DBT assimilating Schala and forming the Dream Devourer. Since we already know that the DBT is a place, well, away from every timeline and every dimension, we can safely assume that once Lavos is defeated in one savegame we can find the Dream Devourer in every dimension- i.e. in EVERY other savegame originated from that one. The DVs may be a gameplay-only requirement. So, we can theorize that if someone else (that isn't Crono's party) want to access the Time's Eclipse, that someone doesn't have to pass through the three DVs.
Another thing: apart from defeating the Dream Devourer, the only way to truly finish the game is defeating Lavos. So, another thing we can theorize is that once Lavos/Dream Devourer is beaten, Crono and friends are "off duty" and never do a battle again (maybe for the exception of the Guardia battle that determines the Rise of Porre). What I'm saying here is this: if we defeat Lavos in a savegame, and then we load back that same savegame, we must think that THAT IS ANOTHER DIMENSION, and that party is another party with another Crono, another Lucca etc. Our "view" of every dimension basically finishes when we defeat Lavos. This thing is fundamental. We can proceed =D
2) Party Magus, while searching for Schala, learns that she's being held captive by Lavos in the DBT and somehow finds a way to get to this place. So, Party Magus enters the DBT (or Time's Eclipse).
~MEANWHILE~
3) Our happy player loads back the savegame in which he defeated Lavos, thus watching another dimension. He finishes the three DVs and goes to the Time's Eclipse.
Guess who's there? Eclipse Magus! But, if you've followed my line, we know that this Eclipse Magus is really our "old" Party Magus! Then, the battle rages: Eclipse/Old Party Magus gets his sorry ass beaten, the alternate-dimension-Crono's party wins but they somehow get their sorry ass equally beaten. Schala teleports all the people out of the Time's Eclipse, probably erasing the memories of Old Party Magus (I don't understand if his memories are erased by himself, actually). She doesn't erase the memories of the alternate-dimension-party: in fact, they were sent to the right time, and when (after some years) Porre+Dalton arrives, the party beats the crap out of them and Guardia reigns.
The history which we witnessed in Chrono Cross (where Porre wins over Guardia, etc.) is simply the history of the Original dimension (those events listed at number 1). Original-dimension-party didn't go to the Dimensional Vortexes at all, thus they didn't know about Dalton and Porre's attack, and thus here's how the Fall of Guardia happened. I think it's a good way to explain the new things in Trigger DS and accept them into canon without having to redo all the theories about the Fall of Guardia and such.
Uhm, well... I think a sort of mini-map can be of help.
Original Dimension, Savegame 1
Porre strikes; party
---------------------------------------------------------------------------------> doesn't know a thing, ---> Chrono Cross Events
| | | | Guardia falls
Crono and party Moonlight Parade The party continues to |
defeat Lavos; events; Party Magus live an happy life, |
Lavos goes to DBT sets back to 12000 BC save for Party Magus |
and forms the to save Schala; which learns that Schala |
Dream Devourer player gets access is kept at Lavos' stake |
to the Dimensional in Time's Eclipse |
Vortexes |
|
|
Party Magus finds a way to the
Time's Eclipse and go there
|
+---------------------------------------------------+
|
Time's Eclipse, Savegame 1*|
|
---------------------------+---------> Dream Devourer events
|
+----------------------------------------------------+
|
Alternate Dimension, Savegame 1* Alternate Crono's party go to the Time's Eclipse
|
--------------------------------------------------------------------------------+--+--> Porre strikes, but the heroes knew and win
| | | |
Instead of defeating They encounter Dalton Gaspar senses something Party is teleported
Lavos, this alternate which tells his dreams at 1999 AD; the way to by Schala out of
Crono's party goes of conquering Guardia Time's Eclipse is open DBT
to the DVs with Porre's army
*note: Savegame 1 is the Original savegame, Savegame 1* means that the player has saved after the credits roll (so, he basically added the DVs to
the Original Savegame).