Ohhhhh sherry, I can see how you desperately want to find a way to not having to put her to sleep.
I am by no means an expert in how dogs are trained when they become guard/protection dogs, but I think Good Hope is right, they need to be stable.
I can see how, if it works, it would give her one last chance. On the other hand, since it is required of her to be "aggressive" I wonder if she would feel at peace, or just feel unhappy and angry. I know I am no help at all, because I would probably sit and bang my head to the wall in your situation.
If I am to advice in any direction, I would have to only and ONLY see to what I would have done, and I would have let Petra go to sleep quietly and peacefully, and not having to go through yet another situation that could possibly make her even more aggressive and unhappy.
Life sucks hun, and I feel for you sooo badly. I would if I were you, want to talk to the people who train these dogs, see what they say, ask the methods they use, and what impact it has on the dogs, and why they want aggressive dogs, rather than stable dogs. Just to give myself peace of mind.
Please keep us updated on what you decide.
I will be thinking about you and Petra, and hope you can come to a conclusion and feel at peace with it.
Hugs
Marit