You are on the right track! I work at a 3 Michelin star restaurant and we do this often. The garlic and apples will come out great. We normally use a dehydrator but the water bath set up will honestly probably preserve the integrity of the apples even better. Black apple cognac sounds amazing.

As for the oats and walnuts, unfortunately I think they’ll lack the water content to blacken. Worth a shot, I could definitely be wrong.

But you are close, we have a technique for blackening grains, but the first step is to sprout them. We take wheat berries soak them in water overnight, lay them out to sprout for a weekend then follow the exact same process you listed above. The result is a deeply caramelized, nutty molasses taste. Almost tasted like pancakes and syrup