Here's my theory of the original timeline, when Crono & Co didn't interfere:
Lavos awakens at Ocean Palace, warps Gurus and Janus.
Janus is warped forward in time to 600AD.
Janus becomes Magus.
Magus summons Lavos.
Lavos warps Magus back to 12000AD.
Magus disguises himself as the Prophet.
Magus gets incinerated or otherwise devoured by Lavos when he faces off with Lavos by himself, when Crono was the one who got incinerated the second time around.
...
As to what happens to the castle, the summoning of Lavos either sploded it, or sent it to another time period.
Think about it; the second time around, Crono & Co are warped back to 65million BC, and Magus to 12000BC. The castle could've ended up someplace else as well.
Magus's Castle might've even also been warped back to 65million BC along with Crono & Co, and the Reptites used the remnants of Magus's Castle to craft the Tyrano Lair.
...
As to what happens to Ozzie in the original timeline in which Crono interfered, either he survived or died. If he survived, he then told his descendants of the great ambitions of Magus. Otherwise, if Ozzie died, then it was only the remaining Mystics left to worship Magus for his ambition.
Remember, it is because of our arrival the first time in Medina, in 1000AD, that we first find out about Magus, and the Mystics tell us his (false) ambitions to kill off all of humanity.
But when Crono & Co interfered, Ozzie must've taken Magus for dead and instead told his descendants of his own exploits.
My guess, the first time around, Ozzie died, but when Crono & Co interfered, Ozzie decided to get away to tell the tale.
