Second layer of drywall on textured ceiling: Do I need to sand down or remove the texture before second layer goes on?
The point of Green Glue is to create a flexible layer of never fully firm glue between layers of drywall, right?  To absorb sound?

In which case, I think you'll need to first level the ceiling, then apply green glue, then apply your final layer.

Otherwise, if you green glue over top of the texture, you'll have high points in the texture that touch the new layer of drywall.  That will transfer sound quite effectively - which you don't want.