Rotobrush

To paint more brush strokes with Rotobrush after it's been frozen, disable Freeze.  

You'll know it's disabled (unfrozen) because the Freeze button will not show as blue.  Also, you'll be able to paint brush strokes.

Regarding propogation of the Rotobrush, it can be helpful to work with shorter clips.  If working with a long clip but only using a shorter segment of it. try exporting a shorter segment as its own clip and re-importing that.  Or, trim it inside of a nested Comp.  Nested Comps make working with the Rotobrush a little trickier as you'll need to open the nested Comp in the Layer Panel.  Hold alt/option while double-clicking the nested Comp in the containing Comp Timeline to do this.

As far as your screenshot goes, it looks like you may be using the Layer with Rotobrush applied as an inverted matte target for another layer.  Unfortunately, it's not so easy to tell from your screenshot.  Check the Track Matte pop-up menu for the Layer with Rotobrush to see if it's set to "No Matte" and for other Layers to to make sure that the Layer with Rotobrush is not a Track Matte Target.  Another possibility (but less likely) is that the background was painted with the Rotobursh. 

As layering starts to get complex, I find it helpful to use the Solo switch to temporarily view specific Layers and isolate what is or is not visible.