No one is going to dump a black box and think they'll get away with it. If the recovery crews find the black box the investigators will know its a diversion when no other wreckage is found, how stupid do you think these guys are? A black box is hundreds of times smaller than other aircraft components that typically stay intact after crashes of this nature. If they find the black boxes first they'll scour every inch of the broader area. Physically impossible for nothing else to be found, they find no wreckage, they'll know by default the box was dumped there.
Another thing, why dump a black box in the first place. If the bogeymen somehow managed to convince them the plane crashed in the area why dump a black box, best just to leave it a complete mystery. If it's a setup, physical evidence dumped may trip up the intended reason for dumping it in the first place.
As for the plane dodging radar. It simply didn't. It was tracked until it reached areas where radar wasn't covered. The British pointed to the area of the present focus because they calculated from equipment pings sent via satellites from the aircraft that's roughly the area it crashed.
Unfortunately the internet has given rise to totally bizarre and illogical commentary. 20 years ago you'd be laughed at in a pub, these days people are free to write anything to a much bigger audience, no matter how ridiculous or absurd it is.
