I understood, and your point is quite valid. However, there is no justification for using Method X should 100% of the coins still be there, never were really gone, and the 850,000 BTC number was a complete hoax. When all the coins are accounted for, it means all of the trades of both Joe and Bill were perfectly "real"
The 800'000 number is the sum of all BTC account balances in the leaked database. It is basically the total BTC that clients deposited over MtGOX's history, minus all the BTC that they have withdrawn.
If indeed MtGOX never had the 600'000+ coins that it claims to have lost, we would have to conclude that the database has been doctored at some point; and in that case it is hard to say that the trades of non-existent coins by Joe and Bill were real.
However, most people seem to agree that the 800'000 figure is correct, and the 600'000+ coins were indeed lost.
The "leaked" database could have been doctored, while the original is still quite intact and unchanged. Perhaps a phony db was leaked out as part of the instigator's plan for covering up tracks. The takedown of Mt. Gox appears to have been well thought-out including a coordinated public relations campaign to convince the public that management had been incompetent and there is a "need" to regulate bitcoin.
Perhaps most people "agree" on the amount of 800K bitcoins, because its what they've been told to believe. The only real sources for this information are a leaked database and the bankruptcy filing which blames the problem on a "bug in the bitcoin system." This latter claim has more or less been proven false.
It's far easier for me to believe the smaller amount of bitcoins. 200K BTC are still worth in the neighborhood of $100 million. Any much larger sum of coins would seem ridiculous for a three year old company such as Mt. Gox.
Of course I am merely speculating, which is exactly my point, even the amount of bitcoins Mt. Gox held is still very much in doubt. There needs to be full disclosure of this kind of information before talking about dividing up "the spoils".