Hi, 

It is difficult to say what it could be from these photos. Wildlife like you mentioned could have eaten any of those, but the chewing on some of the leaves looks like slug or snail damage, possibly a leaf chewing beetle. The diatomaceous earth is probably helpful for any insects or slugs. There are products on the market that act as slug repellent too. You could set up a bait trap with fruit, or look up how to fashion one on YouTube (using an old yogurt or cottage cheese tub), to see if you get a lot of slugs. for any rabbits, deer, squirrels, or other critters that may eat the leaves, either caging or netting the plants would probably be the easiest way to keep them off. 

Just because the dahlias aren't eaten in the front doesn't mean it can't be deer. Sometimes they won't wander into a certain area or decide the plant wasn't very tasty and will move on to something else. 

Rabbits have never seemed to like my dahlias, but I have seen deer eat them before. 

Usually wildlife will leave basil and other strongly scented plants alone, so that is more likely a slug as well. 

Let us know if you have further questions with any of this. 

Emily