Question:

How we know that there were righteous Nephites who survived beyond and separate from the battle of Cumorah.

Answer:

In the 8th chapter of Moroni, Mormon chastises his son for confusion over the baptism of children.  Plus Moroni states that the Lamanites were putting to death all the Nephites who would not deny the Christ. All Nephites at the battle of Cumorah had denied the Christ.