[Amnon]

Statement 1 isn't true. If you have 10$ in the envelope, and it's the smaller envelope, you gain 10$. But if it's the larger envelope you don't lose 10$. The fallacy is that "x" refers to different things in each case.

Statement 2 is true but it isn't relevant, since you don't know the posterior probability given y. If the envelope contains the smaller amount of money then there is no way you can lose -- it's not a 50-50 chance.