Look. We’re arborists. We don’t work in apothecaries. We just work with trees. A rather dangerous profession. Excuse us if we’re a little rough around the edges. The fact that anyone has offered any words at all speaks to the generosity of these individuals who have a lot on their plates to begin with. If the slippery elm bark they sell at herbal suppliers or supplement outlets or wherever are approved for human consumption, I would imagine they’re fine for your cat. I commend you on going to such lengths for your cat but I would have to agree, I think you’re overthinking this a bit. Slippery Elm bark is harvested on healthy, live trees. If you are really concerned about it, go to the harvesters or manufacturers of the brands. They would have more information than we have.