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Why would the Guru's be sent in time if they were not at the palace. The most accepted theory by almost everyone is that Lavos' energy caused the timegates to materialize upon the Guru's and Janus.
Plus, My idea has credit. Senetenal ,you're insane. Zeal never died. She was there all along with Lavos, and then raised the Black Omen.
And don't incine that i'm insane when you have forgotten that the Mammon Machine is still there, and still active!
You cannot disprove or prove that The Guru's and Janus did not appear at the Ocean Palace after Crono and co died. If I remember correctly, someone says that Janus was using Schala and Queen Zeal to get to the Ocean Palace and to Lavos.
Also, because Zeal was ontop of Lavos in Magus' memory, that means that Zeal and Lavos still became friends or whatever. Zeal jumps on his back because she knows that he will allow her. Hence, that means that Zeal should not have died in 12,000AD unless she was forcefully teleported away with Schala's last power. But because this memory also has Zeal on Lavos' back, and she's on Lavos' back when Crono and co confront her, we can assume that it might be the same memory. The Guru's may have followed Crono and co after, and by the time they had got there, it was too late.