Gosh that sounds really hard. I guess you've got to try to avoid anything that would cause him to think that the guarding behaviour is working for him and try to keep all the trials you do 'sub-threshold'. If you walk towards the bowl, he growls and then you give him treats, and which he eats and growls as you walk away, then you are inadvertently rewarding the growling: his first growl, in his mind, has had the positive effect of resulting in treats, and his second growl, in his mind, has had the positive result of getting you to go away and removing the threat to his food.
If he is growling at all, he is above his guarding threshold and I think you are best off aborting the trial altogether - certainly do not give him treats. I think this can be the difficulty with this sort of training, it is so easy to inadvertently make things worse. We certainly had similar difficulties working with Ollie and whilst we do have the Mine book and practice its advice when we know he is likely to guard we don't go out of our way to deliberately test him.