That said, Amplitude Mod as a transmit-receive system is susceptible to being able to detect signals outside the normal operating range provided that they are in close physical proximity,
I agree (as an electronics engineer who at the age of 13 used to build pirate radio transmitters comprising of a pair of 807s, followed by a less than exhaustive PI-tank 
) but I think if this worked habitually, it would soon be realised that the NDB at any airport which also has a VOR there would be pretty useless.
Very interesting about 243 being 121.5. Never spotted that It must be that they simply generated a dirty 121.5 and then filtered everything above 243, to achieve a simple design. 406 is much more complicated in comparison.