I have one issue with GrayLensman's Time Crash illumination; copies of Serge are destined to emerge from the Sea of Eden's Gate Effect, meaning the Serge who returned from the Time Devourer will be Time Bastarded by a bewildered Serge who has no idea what on earth is going on, as to him, he's still in the middle of Chrono Cross.
Perhaps Belthasar did some things to ensure this would not happen.
Entities are not sent into the DBT when they encounter a past or present version of themselves, but rather when two versions of an entity at the same point on their world lines exist simultaniously in the space-time continuum.
At least three versions of Serge exist in the unified timeline:
Serge A - who grew up in the unified timeline
Serge B - who fought the DT
Serge C - who journyed to the Sea of Eden
In 1020 AD, moments after Serge A collapses, Serge B will warp in and Serge A will be sent into the DBT.
Some time later, Serge C will appear outside the Sea of Eden. He exists in the past of Serge B's world line so Time Bastard does not apply. Serge C and B may meet with no consequence.
At the time when Serge B originally warped into the DBT, Serge C's world line will coincide with the past verison of Serge B (just as he warps in from the DBT). Because Serge B is the more senior time-traveler at that point (farther along on his world line), Serge C will be sent into the DBT due to the TB effect.