The initial example presents the same which-part-is-infringing conundrum as the more convoluted example you presented.
- restricted material is X
- generate Y randomly (using a typical cryptographically secure RNG or PRNG, zero-bias, with an entropy source unlikely to be observed by anyone else)
- Z = X xor Y
There is no way to prove that it wasn't Z generated randomly, with Y = X xor Z.
It's clear that the only way to generate both Y and Z is to have the restricted material, but you don't know which piece is tainted and which isn't. You have to know which piece was generated first to know which one is infringing (the non-infringing one had to be used as input for the infringing one).
That just encourages the powers that be to take a "shoot 'em both and let God sort it out" approach. Also, the metadata that says Z = X xor Y will be seen as infringing.
It's clear that the only way to generate both Y and Z is to have the restricted material, but you don't know which piece is tainted and which isn't. You have to know which piece was generated first to know which one is infringing (the non-infringing one had to be used as input for the infringing one).