I see what you're talking about. My confusion originates in your definition of P2, rather than P1, where I thought the confusion was originated.
Suppose two minds, A and B. A has some function for determining truth, let's call it T. Mind B, on the other hand, is running an emulation of mind A, and its truth function is not(T).
Okay, yes, this is an utterly pedantic kind of argument, but I think it demonstrates that in -all- of mindspace, it's impossible to have any universally compelling argument, without relying on balancing two infinities (number of possible arguments and number of possible minds) against each other and declaring a winner.
That sounds pretty good to me, though I think it's an open question whether or not what you're talking about is possible. That is, a UCMA theorist would accuse you of begging the question if you assumed at the outset that the above is a possibility.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.