Dead Pokemon

[Infinity Mk-II]

Shedinja IIRC is just the empty cocoon or pupa or whatever you call it of a nincada that turned into a ninjask. Probably some leftovers of the memories and experiences stay within it, and a free, actual ghost, seeing a free spot in your party decides to join by possesing the thing.
As a result of the cocoon probably having still alive secondary organs (which were previously developed and used by the nincada to become a ninjask), the ghost manipulates them so they don't die like they normally would. This also would explain why it only has 1 hp as these organs won't survive on their own and the ability as the ghost wouldn't let its almost-dead body to be damaged.
Of course, with a full party there's no room for any ghost. So the thing just becomes useless.

 

[The Outrage]

I was thinking about Shedinja, before I evolved Nincada, I thought it evolved like Wurmple, only becomeing one pokemon, luckily I tried evolving my first Nincada into a Shedinja, was disapointed when I saw Ninjask, even though now I know that Ninjask is the stronger one, it is for sure Shedinja is the harder to get, then I noticed that you get Shedinja if you do it correctly as both of them were on my team, iot only shows it evolving to Ninjask though.

Anyway, on topic, I had a theory about Shedinja as a ghost type. Not sure if this has been posted before. In the real world, there are types of insects that lay their eggs inside another insect, the larva inside, is a parasite that eats away the inside of the host insect, eventually it dies, and the parasite insect the host in it's adult form, I think its like a type of fly. My theory is that a Ninjask lays it's egg in a Nincada, if the Nincada is allready in a pokeball, the Ninjask larvae must also be in there, as it kills the Nincada when leaving, it becomes the only one in the Pokeball. The Shedinja, on the other hand, is the Nincada that died, but because the ninjask has taken it's place in the party, it can only join the party if there is room for it.
Not sure how that theory sounds, basically, Ninjask starts as a larvae parasite, it kills Nincada and hatches to the adult form of Ninjask, Shedinga is the ghost of the Nincada.

There are holes all over your theory, especially if you consider the fact that Incada are basically the larval form of Ninjasks.

Shedinja is not the empty cocoon, but the emty shell of a Nincada about to evolve from Ninjask. It's most likely the energy given off by the evolution of Nincada to ninjask that gives it's empty shell new life.

If a soul does exist, and Pokemon do have them, then a Shedinja most likely has a fragment of the Ninjask's soul to keep it alive. Between the two Pokemon, they make a perfect duo to keep each other alive. Shedinja can shield Ninjask from most hits with it's wonder guard, while Ninjask provides speedy transport for it. This "fragmented soul" evolution seems more like an adaptation than anything.

This also leads me to believe that Shedinja itself is not actually an evolution of Nincada like Ninjask is. It is merely a leftover product from evolution. It is more like a spawn with identical DNA. (when I tried to say this on SPPf they just kept saying it was an evolution because the Pokemon cards says so)

 

[Swampert tamer]

I guess it's obvious that pokemon in fact die because in the games the Pokemon Tower and Mount Pyre are like graveyards, but they don't use that word, but say that the spirits sleep, etc.
What happens when they die? After the pokemon tower incident it is said that Marowak went to the afterlife...