Interesting...is there any reason for him not to exist? I guess it depends on which of the Dimensional Unification theories you believe. I honestly don't know, but Guile (post-Dimensional Unification) should have no holding on Magus as Guile--it's just his ending...
If Guile is Magus, after the Chrono Cross is used and all the other dimensions are merged into Homeworld and no one has memories from any other time lines, he's the only incarnation of Magus left.
Also, having just beat Chrono Cross, I'm of the opinion that Lucca was killed in the fire when Lynx attacked and something tragic befell Crono and Marle since Lucca says. "Sorry for making you sad, Kid... You see, we no longer exist in this timeline."
And as far as Magus being trapped in 12,000 BC, there's pictures of Frog, Lucca, and Robo on the walls of the orphanage. There's no reason Magus couldn't have also made his way back to the present as well. And actually, I've been thinking that it makes more sense for Janus to be Guile rather than Magus because Guile is 26, the year is 1020, Janus was about 6 when he fell into the dimensional vortex. By my count Magus was about around 30 when you fight him in 600 AD. Even assuming he traveled to the present he would be pushing 50 by the time Guile is supposed to be 26.
Also, I found this in my database from back when I was much bigger into the series:
MAGUS, JANUS, GILBERT, AND GUILE EXPLAINED
Intro
First of all in the end of CT, during the new FMV's Magus is shown going into the forest to search for Schala, but the game never says anything else. Unfortunately it took the script writers to get a clear explanation of what happened. Magus goes off with Alfador, who seems to ignore everything and heads to look for her. However Magus says he is going to go back into the past to find Schala. The funny thing is what the scenario writer says on it is very confusing: "While we intended for Guile to be Magus, we did not make this intention clear enough and changed his clothes to make people no believe he was Magus. Guile is actually based off a character in Radical Dreamers which is believed to be Magus, and guides Kid through the game. However while we made it so Magus was to be the one doing the time hop, we decided to rework the events so that Janus was the one. This is how the area of Radical Dreamers turns out in the end." While he states they in fact intended Guile to be Magus, the way they word it is not something that any right hand gamer can just off the fly understand. When decoding the real wording and intention, here is what so is: During the time Magus left to go search for Schala, was during the exact incident where Janus is supposedly going to go through dimensions crossing over. However, when this event transpires, it is at the exact same moment which the Time Crash occurs. Janus instead who can not time hop, does get spliced into the dimensional vortex creating a rupture which makes several unconscious Time Doppelgangers, which are not aware of anything because those dimensions themselves have been created. The same time Janus stepped through the portal, all of a sudden the Radical Dreamers dimension was created off of an alternate split reality. Janus then was lost in that space of the moment, creating an alternate time doppelganger in a reality never his own. Because Janus in this instance never went through the dimensional gateway, and did not ever become Magus later on, he stays the same.
Guile is Janus (Time Crash/Memory Loss)
Therefore Janus of Zeal, who at that moment of the Time Crash, was lost into another being. Alfador on the other hand did not travel anywhere near the present time, he instead got lost in the chaos. The other being washed up on the shore in 1000 AD with no memory whatsoever of his origin, a "spawn" of another life form. This being quickly became a traveling magician, called Guile by the people of Termina. He did wacky fortunes, and told funny stories while he collected his weight as a magician to take on an even larger stake: That of discovering the truth which was hidden from him like a black veil. He learned the trains of a magician, however, knew no such special spells, only little magic tricks. He met up with Serge and co who were off to defeat Lynx, and needed something to pass the time. He went with them, never deciding to take off his mask because he liked his enigma qualities, and when he reached the Frozen Flame, he stumbled back. Unaware that such a power could have existed in him, perhaps, but never will. He exclaims in the game if you have him in the front of the party "I am not that ...No....It's not possible....I am not that!!!" and indeed he is correct, he is not that. Guile is a rip-off time doppelganger of Janus; he has no correlation to Magus whatsoever. He only had the appearance of that of Janus, not Magus, nor even knew what the Mystics were. He was born, and bred, an idiot. After that event, he sought to repay his gambling debts Perhaps Sneff knew what his life was really like. Hmm??? Or perhaps the mask he takes off in front of Kid which produces no reaction when he loses his 'bet'. Guile always wished to be something he's not, something he could never be.
Gilbert is Magus (Time Crash/Memory Loss)
At the same time Magus was in that moment traveling backwards across that plane and in the Time Crash spawned himself into another "life form". This life form was simply known as Gilbert. One day Gilbert arrived in the form of a young earnest side show traveler, who was quickly adopted by parents, met his girlfriend, and became a great thief. He went on through his life to discover he might hold untapped powers, ones that could shake the heavens. Or maybe he just liked his life as a thief. He decided one day long ago that he would go on an adventure, and with Serge and CO joined the Radical Dreamers to become part of this adventure and live it to the last day. But when they traveled into Lynx's Manor, and he had grown attached to his friends, he realized the truth about his life. Something he could not ever attain, something he could not ever be. He wished to be something more than a thief, who traveled and stole the Frozen Flame. Perhaps the first thief to steal the Frozen Flame??? Or maybe the last??? Still, he dreamed of fighting the wars of chaos, and learning from his shadow powers he inherited automatically from his real birthmother(who lies across dimensions) decided to make his life the fullest it could possibly be. He went on and wished to marry Lulu, they had several daughters, and in that life, lived happily and as rich as can be. He could never ever say that he knew what the strength of his abilities were, but he traveled the stars and cosmos, running across mischief and adventure, where he styled his name after a rock star, and became well known throughout for hunting space criminals. He never knew the truth which birthed him, he never could accept that. He instead gave his life into the card tricks which a thief will play, and learned and as a wandering thief he was remembered by those who cared in the land.
Outro
Flash forward to 1020 AD, The Chrono Cross is used. All of a sudden every dimension which existed outside of home ceases to exist, Guile, Magil, and all incarnations of Serge, General Viper and every character are wiped completely out. They were never born. Just as in sliders if you were to look at one of the dimensions where Quinn found out he discovered the theory of relativity. It never happened, he isn't that smart, and that dimension after everything else ceased completely. Because of this, one can not say that Mars Daio, the Old Woman, or any of them were in fact real. As Kid learns, at the end of her story, she will never be a thief again. And in that sense, she cannot. After all, Magil, Serge, those people are who made the game fun. As for the cat Alfador, he does nothing but follow Magus, who is at this moment, unaware of everyone, dwelling in the past looking for Schala, and watching over her through the dimensional gateway he travels. He knows nothing of the strange bygones, and may, as it were not ever realize what truly happened. Yet in the same sense, his play in the role of the events caused those with imagination to wonder, would Magus really come back??? The answer is nay, not until the writers of the Chrono Sequel decide to put him in. Though maybe everyone's favorite Mystic raised brat is not present, the world goes on. And in that same sense, people in the games like Glenn and Frog, are distanced by hundreds of years making it impossible for them to even have the same bloodline, much less exist in the same moment. However, those who will dream, may say some day the fateful Frog returns to his human form, and comes back to the world to slay Lynx. There is nothing wrong with a dream.