re the ANIMAL DIGEST (yeah, ewww! ), the way I understand it is carnivores benefit from consuming veggies that have been partially digested by their prey(ie--the contents of a deer's intestines) because they lack the enzymes needed to break down veggies consumed directly by themselves.
For example, if you feed a dog a raw carrot, a chewed up raw carrot comes out the other end. Their short, meat-eating tailored digestive systems can't process the veg. At the other end of the spectrum, cows, who eat only veggies(grass), have not one, but four stomachs. Looooong digestive process converts veggies to...well, you know.
I'm no expert in the field but had to do a little research when I switched our three dogs to a homecooked diet. Nothing says love like sound and smell of pureed liver in the blender.
But, like you, we do what we can to keep them happy and healthy for as long as it is in our hands to do so.
Best wishes.