There is no UI to manager tilesets, anyway I guess that these kind of tilesets that are not associated with tilemaps should be removed automatically. Taking note about these two issues (anyway the tileset management is planned).
Yes, we’ve used Rotation angle and Horizontal skew angle, but the fields are too confusing to handle a vertical skewing manually/by text fields. So we’ll try to add the vertical skew angle too in the fields.