2) I don't argue that a basin shouldn't be mapped with a "natural" key. I only feel that the prefix "disused:" isn't really appropriate for the "natural" key. The prefix "was:" is more encompassing, why not to use it in such cases? But I don't have a hard stance on this issue.

3) "brownfield" is pretty much anything that one was developed, but isn't anymore (and hasn't become totally overgrown as yet). A typical example is a once paved surface with some rubble and some grass or shrubs starting to grow from it, — that is, just what the basin floor are now. After some time, it will become natural=grass, but as yet, it's just a waste of space. And no, there do not need to be any plans for redevelopment for an area to be a brownfield, it's just a present state of land.