They are different things entirely. Render extents kinda define the extents of your world. By default, your main extent is your world, and you build it into "tiles", it just gets split into multiple pieces.
So in short,
1.) No, one does not negate the other, and
2.) Like I said, render extents have to do with your current world. So if you created a custom extent to isolate, say, a single mountain from a bigger terrain, say, an island, You will only split that one mountain into 8x8tiles on export. Make sure your whole terrain is your current render extent. If you did not change the extents anyway, just do nothing with regards to that. Just edit your tile settings and build tiled world.
The purpose of render extent is to go deeper and create most complex terrains you can imagine. It's like a "focus tool", so you can look at a biome closer. And it's used by the layout tool as well. If you don't wanna get into those, don't. It has nothing to do with tiling.
