Tricky. Our small, rescue terrier was so badly treated before we got him (we didn't know how bad he was) that he has 'fear aggression' with other dogs and is suspicious of people until he gets to know them. We've managed, and he's lived a happy little life, although keeping us on our toes, but you have extra size and strength to cope with.
I don't think you need to worry about a garden as long as you make sure that it is really dog-proof.
She was missing you and attention and love during her growing spell. Your dad obviously didn't have time for her. To remedy this is going to take time.Don't worry too much about the killed hen etc. in the past - one of my sister's very well-bred & cared for labs. couldn't resist a fluttering bantam when young.......... but soon learned to be bomb-proof. Again, it comes down to training.
All is not lost, but she needs continuity of care and training. How can you only visit her every 2 months if you want to keep her in a garden? Sorry, may be me just not understanding.
You definitely need professional help with her i.m.o.. Good luck to both of you. :)