LEOs do have some immunity, but a plaintiff can get around it if he can show that the LEO’s conduct was particularly egregious. I usually don’t find Wikipedia to be a good source for anything controversial, but it has a pretty good explanation: http://en.wikipedia.org/wiki/Qualified_immunity.
By the definition outlined in your reference, I think Ms. Mosby is in deep do do. Especially since she didn’t run this through the grand jury, but acted on her own.