Most clients now support outbox, so you don't need a common relay. Users have inbox and outbox relays, and clients use these to retrieve and send notes.
Animals absolutely do synthesize nutritious substances that you can't get out of plants. For instance humans, as long as we ingest the 9 essential amino acids, can create from scratch any protein our body needs. Our bodies are also capable of turning protein and fat into carbohydrates when needed (https://en.wikipedia.org/wiki/Gluconeogenesis).
Animals, us included, are not inefficient filters, or tanks that hold minerals and energy. We are chemistry sets designed for survival in a wide range of conditions.
