I think there is a combination of problems.
You say you can train a dog and the dog is fully trained in another thread but the dog doesn't let go of something when you tell it.
That means the dog has a very limited training.
An off command is a very basic command. Considering the dog has this drive to hold onto objects i feel you can bridge the gap of her fear in the garden using her prey drive. But you need to research much more and spend a lot of time with the dog and find good ways to exercise and toys which she is attracted to and ask for obedience in return for her getting her toy. Once you get an off going you can use tug to train her as well.
I don't know what your club or old trainer is like but I don't like the sound of a place where dogs can fight and have so much contact with each other. Also the trainers dogs should be very well trained considering he is the trainer conducting classes.
The dog being afraid of objects is a sign of weak nerves and environmental stress. A confident dog would burst the blow up toys. I feel a confident dog would also not be so aggressive with strangers. A fearful dog will be nervous of a plastic bag floating in the wind.
The reaction with strangers can be a sign the dog is feeling insecure in it's home due to nobody successfully taking on the leadership role and effectively controlling the dog and peoples movement withing the environment. Sometimes through body language you can show the dog you are in control and instruct it to act as you like. A place command is good for starters. Any time the dog gets aggro or excited tell to go to a spot and stay there.
Here's a good site on explaining environmental stress and how to deal with it
Desensitizing and Counter-Conditioning: Overcoming Your Dog?s Issues
Also it can just be the temperament of the dog. Generally I feel it is a combination of upbringing/training, temperament and early socialization.
I have a female non gsd(bullmastiffX) which was human aggressive and environmentally weak nerved. With a lot of training and counter conditioning and with age this dog got much better. I found a good club for training and put in a lot of hours myself and tried different techniques.
She shows no sign now of fear of objects or people and shows no aggression with people but still is alert and on guard when necessary. She shows a lot of guardian instinct but is balanced and very stable now.
Check these vids on how to walk your dog properly and how to use the leash to control the dog.
Tyler Muto - YouTube
You also can pick up good tips with Cesar's way and CM's tv shows. I find his techniques good for having visitors, getting dog used to people and fearful or excitable dogs. A lot of good hints on how energy and body language is important in dog handling. He has some simple but important rules and guidelines on how to manage a dog.