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Honestly, I don't even know what "richness" means. Is that supposed to be protein content? I really don't buy into that idea too much. When Nemo had severe intestinal issues (likely intestinal cancer) which led to chronic diarrhea we were able to get him to do well on raw food for a decent period of time which is relatively high in fat and protein (I have an integrative medicine vet) where the specialist recommended the "highly digestible" science diet stuff. He had some food sensitivities before that, likely chicken (not raw but in processed foods) and corn. If your dog was having food issues before the incident then I would suggest doing an elimination diet and try a novel protein source and see what happens. I've been feeding my dogs the Sophos complete dehydrated raw food when travelling the past few weeks and it has worked well. You can buy small bags of it to try it.
The research with probiotics and people isn't really conclusive or straight forward either but many find benefits with them. But it's one of those things that might help and really can't hurt too much other than your pocketbook and maybe some gassiness with your dog.
Which ID food was it? What was the protein? Which Fromms were you using? In this case you may just have to try a less than ideal food but better than the ID to get him more normal and then transition back to a better food.
One thing to consider is the Nutriscan test (I haven't looked back in the thread to see if it was brought up before). http://nutriscan.org/ It's a bit pricey if you do both panels but it might indicate if he has sensitivities to any particular things and then you would know to avoid them in diet selection.
And a trainer can help you set up the situations to desensitize and countercondition if you aren't able yourself to do it. Just be clear in what you want from them up front. You want modify his behavior not just manage it. Although you will always have to do some level of management.
Read Jean Donaldson's "Mine". There is a protocol there for resource guarding. It mainly applies to humans in that book.
For now you have to manage. So crate the dogs while they are eating, even if it is a treat. Or at least one of them. And no one can give him food except you. You will just have to be mindful of that.
Starting just by yourself with Oakley is how you should start with any of the behaviors and then move up to having a trigger present. You can certainly make the situation better but I don't know if you can completely fix it, that is likely dog and situation dependent.