What the trainer did was just plain silly, and your gut told you so. Listen to your gut. If it says you see something nonsensical, then it more than likely is.
The owner already has control of the food. Have the dog sit, make eye contact, wait for a release word before he can eat, and that is enough to make him do to understand that it is your food and you are letting him have it because he worked for it.
I have kids so having a dog who would not be food aggressive was very important to me. All of us have, at times, gone over to him when he is almost finished eating, and added something really great. Now when we approach as he eats, he wags his tail.
Simply thinking like an animal can give you ideas on how to establish trust. It doesn't have to be so complicated.
Going back to your first question about not eating, well, it could be that the BB is not making him feel very good. I have heard people having problems with it being too rich for GSDs. They tend to have sensitive stomachs and a food they have been eating for a while with no apparent problem can, with time, start to bother their gut. This happened to me, and I had to change what I was doing, and I was, and am, feeding raw.
You just have to find what the dog enjoys eating and does well on. Look into a different food. If a puppy isn't eating there is a good reason.