How to create mirror ROI in the other hemisphere?

Is there a way to control it so I can make it as similar to the scout in the original hemisphere?

Not at the moment. But I guess this is possible using the FreeSurfer registered spheres (http://neuroimage.usc.edu/brainstorm/Tutorials/CoregisterSubjects). The right sphere could be mirrored to match the left sphere, and then the region could be projected from one sphere to the other, like it is done when projecting scouts across surfaces or subjects (brainstorm3/toolbox/math/bst_project_scouts.m at master · brainstorm-tools/brainstorm3 · GitHub).

This is already part of our infinite todo list (http://neuroimage.usc.edu/brainstorm/Next#Anatomy).
We currently do not have the resource necessary to develop this feature, but you are more than welcome to contribute to the project and help us developing it.

or is there a paint tool to manually erase and add vertices within the mirror scout?

Yes, there is: read the tooltip of the first button in the panel for instructions. Or read the instructions in section "Add vertex manually" in the tutorial:
http://neuroimage.usc.edu/brainstorm/Tutorials/Scouts#Growing_a_scout
Click on the "Edge" button of the Surface tab to see the vertices that you can actually click.