"They did seem very sure that they had located a ULB though, in which case, your original point would still stand"
correct. lets forget all handshakes and inmarsat - when you receive in the middle of the indian ocean where no other aircraft crashed short range stationary pings from a device that is confirmed to be a blackbox ULB which start to fade away just 30 days after mh 370 vanished - what else can it be than a malaysian boeing 777 directly unter your feet ?
this pings either never existed or were from another source OR bluefin is not able to fully scan the bottom because otherwise MH370 must have been found .
Pardon me if this has been discussed - I gave up reading this thread midway due to so much uninformed speculation - but iirc from the AF447 threads an acoustic wave can be bent as it passes through the interface between media of different density (e.g. layers of different temperature) and thus peak detection would occur when the receiver was not directly overhead the source.