Slats11, some nits
Rather than a straight line approximation look into a second or higher order curve that better fits. The maps Saturn has put up show weak currents to the NE that turn into slightly stronger currents to the N and then even stronger currents to the NW in that region at that time. I'd expect them to have gone down to the West of the over simplified straight line estimate.
Fernando de Noronha is to the SW and has roughly a mile of runway. So that part of your comment raises a huge, "But, WHY?"
Regarding life jackets, I've noted it's really hard to reach that silly object in its storage under the seat cushion while I am strapped in to keep from kissing the overhead due to turbulence.
Pinger range is very short compared to the depth of the ocean where the plane went down. As you mention multiple likely thermal discontinuities make it even less likely a mere submarine at even twice its Wikipedia estimated maximum survivable depth would get close enough to hear the pinger.
Missing radio is a little unlikely. If the current setup was for transmit on DAKAR's frequency when the microphone switch on the joystick was pressed that could explain a lot. While the world is turning to (oops) around you reaching to reset the console to transmit on some other pre-tuned radio would be unusual. So I'm merely uncomfortable with missing radio calls, well before the situation turned bad. Once it went to perdition setting up to transmit a mayday on 121.5 or the like seems somewhat like the little mouse raising its middle digit as the owl swoops in for dinner.