There is nothing magical about the 51% number. If 51% were to chose to give the BTC away (taking it from the FBI), first of all it would require these 51% to do

There is nothing magical about the 51% number. If 51% were to chose to give the BTC away (taking it from the FBI), first of all it would require these 51% to download and run new Bitcoin software that recognizes this BTC gift. Then you would end up with a network split, a chain fork, where 49% would not recognize the gift, and 51% would recognize it. If some vendors or exchanges happen to be in the 49%, then whoever received the BTC gift would NOT be able to send the BTC to these vendors or exchanges. You are confusing (1) a majority attack (mining blocks following the existing rules) that has a certainty of success with more than 50% of the mining power, with (2) a majority of users trying to change existing rules (in which case you need consensus = 100%).