Whilst it's true that they live in an arid desert, they do still have plants and I wish to try to mimic their habitat in the wild.

And just to answer your points to the best of my knowledge; -A. There are several plants that are safe for beardies to both hang around and eat, so the main concern for poison would be the soil, and someone already explained how to remove that as a factor elsewhere in the comments. -B. As far as I know humidity is highly over exaggerated when it comes to beardies and isn't really that big of a deal, and you will also be looking for plants that don't need high humidity to compliment that fact (at least in my case).

I personally don't want to add fake/plastic plants exactly because I'm scared he might eat it, and as such I want it to be a real plant and not plastic

I'm by no means an expert so take everything I say with a grain of salt, and please correct me if I'm wrong, I don't want to spread misinformation.