I don't think this is an easy one, and with the pup coming to stay for a bit it might be as well to just avoid the situation and feed them in separate rooms. Or with Oakley in his crate. (you could make "crate" or whatever word you use as a cue he is going to get food.)
I am not sure what approach I would take at this point, but I definitely would not allow anyone to feed him treats. I see this as a potential problem with dogs trained with food rewards. There are always some that will be greedy and want any food they see, especially if another dog is getting it. It's also one reason I am dead against ever letting a dog accept food from strangers, especially children! (although some people advocate this as a way to accustom shy dogs to other people). You set your dog up to expect that any visible food may indeed be for him. I think you want to send the opposite message, so perhaps no food at all except in specific circumstances. If he doesn't expect to have any of it, perhaps he won't feel compelled to guard it…....but at this point it will be a long process to turn around.
Growling at people should attract consequences. However, this can be tricky because with a dog that is already showing aggression any disciplinary action could exacerbate the problem. Without knowing your relationship, it's hard to determine the best course of action. Ignoring the growling (and him!) can work, if he understands that such behaviour gets him ostracized for a time. If I were preparing food and he growled (at anyone!), he would obtain no food from me, even if it were his customary mealtime. I would wait a bit, then possibly send him to his crate or mat, order him to stay, then if he complied I would bring his food to him there. Make his eating place his own private spot, and don't feed him anywhere else.
And perhaps lay off the treats in training. At home, you can still click with a delayed reward. I do it all the time. Get the behaviour I want, and hand out a "promissory note", and quickly go to the "food" area for the reward.
As far as a trainer goes, this would work best in a "live in" situation, if you know someone you can trust. The problem is, the trainer may get wonderful cooperation but not manage to change how the dog is when he is home with you!