The system was set by the copilot. That would be your agent. Prove that he was an agent, and you've succeeded.
However, this kind of malfunction is ridiculously easy to induce on INS systems where the user manually punches in data on a keypad--I am amazed that this didn't happen a lot more often than it did back in the Bad Old Days before GPS (or even data entry from a preprogrammed tape). I go with Occam's Razor on this one: the simplest explanation is "human screwup." Also, if the plane landed somewhat intact, then you have a hard time explaining the wreckage that WAS recovered, and the passengers would be dead almost immediately on arrival (you cannot take ANY chances that one might escape--one LIVING passenger or crew member making it out of the USSR would spill the beans).
The argument presented for this case relies on numerous nonfactual assertions (it happens to require that the Earth be flat).