Lets commenters add a drawing or doodle on a picture from a given post.
No. That would be extremely tricky. Maybe later.
Sort of. The eraser button puts you in eraser mode, with the eraser size based on the same width buttons above. But all you're actually doing is painting a white circle on top of your art.
You may need to manually set the name of the class your theme uses to designate a post. See the advanced options in the admin area.
I've seen this happen in Firefox if you're logged in as an admin, but there may be other conditions where it occurs, especially in the myriad themes that exist. Is it happening to your users, or only to a logged-in administrator? Maybe it's nothing to worry about. Otherwise, email me.