The mipmap option on the sprite collection is applied to the texture. A sprite collection can have more than one atlas texture, thats why the setting is on the sprite collection. Just set it on the sprite collection, it gets propagated to the atlas textures.
The rescale option is an experimental feature to rescale the textures by 0.5 or 0.25x, its not meant to be enabled in the GUI under normal use. I'll add a note to the docs, thanks.