Your theory is close to mine - after the Emperor knows that Lulu=Zero and that he got Geass from CC unconfirmed spoiler came out, I thought that maybe when the spotlight is focused on Schneziel and everyone saw the potential for him to become a great ruler, the Emperor saw something in Lelouch that convinced him that Lelouch can also become just as great, and maybe even greater. He wanted to fully explore Lelouch's potential, but Lulu's just accepting and going along with all those political conflicts and the emperor decided he should pull some strings, which led to the assasination of Marianne. The emperor is certainly more than a trapezium organ.
But still, I would find it rather cliche and disappointing if the final showdown between Lelouch and the emperor turns out to be like this:
Lelouch: Why did you kill my mother? Emperor: To give you a reason to fight.
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But still, I would find it rather cliche and disappointing if the final showdown between Lelouch and the emperor turns out to be like this:
Lelouch: Why did you kill my mother?
Emperor: To give you a reason to fight.