Hi, sorry to hear you‘re still having problems, I do hope the vet will be able to help you get to the bottom of it and that Chester perks up soon. I don‘t have a lot of experience with guarding but can‘t really agree that they tend to guard food rather than toys, that isn‘t the case with Humphrey who would let me take his bowl away while he‘s eating but can sometimes get very grumpy if I want him to give me his ball. I think they guard different things, places, things, people etc depending on their value to the dog.
You sound a little stressed about the situation ( that‘s just the impression I have but please forgive me if I‘m wrong) and I‘m just wondering if your anxiety is rubbing off on Chester and making the situation worse? If the vet‘s tests don‘t reveal anything that explain the loss of appetite, and he‘s physically ok, I would suggest putting his normal food down in the normal way and see what happens. I think if you go down the road of hand feeding and trying to tempt with this and that, you‘re in danger of creating a fussy eater, you‘re also sending out the message that you‘re worried and that might unsettle him even more.
I hope you can resolve the problems soon, best of luck!