OMG,
With this theory, ISN'T IT HARD TO UNDERSTAND THAT MAGUS DIED AND HE WAS REBORN INTO SERGE! It isn't that hard to understand.
Even if Marge and Wazuki are his parents, it doesn't mean he can't be a reincarnation of anything. Like my past reference to Buu in DBZ. He was wished and reincarnated into a kid called Uub, but some of his power he kept.
Magus had powers to predict or foretell death, right? So did Serge. Theres one thing down. They both have blue hair.. Okay.. Both have something to do with Schala or Kid....
Also, if you're reincarnated, you won't turn out the same way. Like they say, a child i based 25% on their parent and 75% on their environment. If Serge is Janus, then we see that he grew up as a nobody with a peaceful life, away from magic and something controlling what he does. Even though FATE controls people, it just puts an idea into their mind.
I won't go any farther, but you can see that it's possible that Serge was based on Magus, but not him.