The natural explanation assumes either To reduce things to basics, the proposed methods for the medium cheating are:
1) Eliciting the aid of other chess players.
2) Extensive research of previous games & chess literature.
3) Computer assistance.
4) Combination of the above.
The computer angle seems the least likely. It wasn't until 1988 that a GM even lost to a computer. The computers that were playing high-level tournament chess in the late 80's-early 90's were sponsored by companies like Cray and IBM. I don't remember there being anything you could buy in the store that was rated over 1,800 at the time.
But yes, the natural explanation seems to require the aid of not just a chess player, but a chess player capable of making a GM doubt the outcome of the game. If Korochoi and Helmut Metz are to be belived, the mysterious chess adviser would also have had to make his play appear old-fashioned. Some have suggested that Korochoi didn't take the game seriously, but we have no evidence of that. All we know is he was ranked 3rd in the world at the time, and stuck with the game for 8 years. In the absence of any other evidence, it's reasonable to assume the mysterious chess player(s) was very highly rated. So somehow, either Eisenbeiss, the medium (or both) had access to a highly rated player(s) for 8 years, who would have kept quiet 15 years after the game. It's certainly possible, but how likely is it? But that's not the most implausible part.
The natural explanation assumes at least Eisenbeiss or the medium (or both) set out to perpetrate a hoax. From what we know of elaborate hoaxes, they are done for one of three reasons: Profit, publicity, or the thrill of pulling it off. What else is there? I was bored so I decided to play a correspondance game with a GM for 8 years, fly to Hungary, do 70 hours of research and interview some dead guy's family? As there was no money at stake and the game garnered virtually zero publicity, the only plausible motivation is to see if it could be done.
So I think the naturalistic explanation is weak on motivation. This kind of hoax would take quite a lot of work to pull off (finding a high-ranking player to advise you, staying with the game for 8 years, and quite a bit of research). If the only reasonable explanation is "Hey, look how I fooled everyone", why did Rollans go to his grave with the secret? Don't people usually brag when they pull off a big hoax just for the hell of it? If Eisenbeiss was the mastermind, why has HE kept quiet all these years? HE certainly gained nothing from the game. And if its all a fake, why drag the game out for 8 years? The hoax would be just as valid if the game had finished in 1 year, and that would have left plenty of time to perpetuate OTHER hoaxes on unsuspecting GM's. But no attempt was ever made again.
So far, you have not provided us with any compelling argument to believe that any of these methods are unworkable. (At most, the valid portions of your arguments show these methods would be difficult, but not impossible.)
I never said the natural explanation was impossible. I pointed out that it was weak in certain areas, namely motivation and lack of explanation for the length of the game.
Also, many people have gone to extraordinary lengths to perpetuate hoaxes with no apparent motive for doing so.
There's always a motive. Again, did Rollans or Eisenbeiss decide to do all this just out of boredom?
As the medium in this case had ample time and several possible methods of faking it, do you really expect us to accept this as proof of an afterlife?
I never said it was "proof" of an afterlife. I said it was "interesting", and I still think so. I think the fact that someone went to a lot of trouble over many years for no material benefit (and died with the secret) raises a lot of questions.
I think Harry Houdini had a good idea for a test of a medium's abilities.
http://en.wikipedia.org/wiki/Harry_Houdini#Code_words
If the test had been blind (ie, his wife didn't know the code words in advance, and had to verify them with a trusted third party not present during the readings) it would have been perfect.
Is there a single case of Houdini exposing a medium who worked for FREE?