Per World Settings

When creating per world settings do I need to copy the entire default world settings and rename it as a child or do I just add a child world name after the default settings and the settings I want to override?

No, you only need to add settings that you want to override for that world

I would like to add the world nether but not sure what to use for the following. This is what I have so far.

hidden-blocks:
chest
spawner
nether_quartz_ore

replacement-blocks:
gravel netherrack

Any other suggestions?