The destruction of King Kynd was by throwing him into the bottomless chasm.

The destruction of King Phearse was carried out by sinking him into a deep pool, down in the bottom in the mud, held down by stones so he could never escape and was of no more use to himself or the world than if he had died.

It has already been indicated by Jinjur and Glinda that to have the right to live, one must be of use.

The basic moral ethic of the Ebionites is Recursive - non preferentialized Good and potentialized Evil. Ref Emerald City part 3.

The basic moral ethical code of Oz appears to be a Recursive Utilitarianism. For things which are just things, to be of living use is to have the capability to live in the first place. For those who are alive, to be of use is to have the right to continue to live.

To lose usefulness is to cease to be effectively alive. Falling forever, smushed down in the mud, they are destroyed. If they are not rescued in a brief period of time they are lost forever for no one will go looking for them because they cannot be found. In an actively Recursive world, if they could be rescued, the potentiality of their being rescued would keep them in people's minds. If they have no such potentiality, their story is over and they are lost forever and nobody ever worries about them ever no never again. It's that lack of worrying that indicates that their story is over.  It seems to me that the only reason that Ozma was rescued was that Glinda never ceased worrying about her.

Concerning the invisibility of Oz. From Mo they fly over the Deadly Desert. Trot is fated and Button Bright is able to get lost to anywhere which is why I think that they enter a pink mist and find the Quadling country. If it was just the Orc, the birds and Cap'n Bill I don't think there would have been any mist and they would just come out on the far side of the desert.

Leaves make a warm blanket? Maybe in a fairy country. In my experience camping and inadvertently camping, fallen leaves are pretty near useless for anything except to be stuffed into bags and make crunkle noises that keep you up all night.