has she always lived outside or is this new? Labs are family/pack animals and do best in an environement where they spend most hours with their family. Unless you work outside with your dog it is impossible to spend the right amount of time with a dog when they live outside, even if you spend 2-3 a day outside with them (which is alot for a busy family) that leaves 21-22 hours where the dog is ALONE.

You really need to have her looked at by a behaviorist, not sure if any of the people you mention were or not. And it takes special qualities for a dog to become a working dog so at best5 that is a long shot. if it worked out awesome but chances are small (not saying your girl is not smart or anything, just that VERY few dogs pass the programs as few dogs have teh natural ability (even with training).

lastly - be honest with yourself (and everyone in the family should be). If you hold any anger or hard feelings towards the lab, any at all (as would be understandable) you need to rehome her. She needs a home that doesn't hold a grunge against her (again, this would be understandable in the circumstances). only keep her if you love her and hold no ill will. Also, if you become stressed whenever she meets another dog after this (or other person, child, animal) the dog will FEEL that and it will impact their reaction. Dogs pick up on our vibes and feed into it.