"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 .