encounters 1 to 3 the plane seemed to be moving south at about 1 knot
.
Maybe it was floating at some great depth or tumbling across the seabed
I really doubt this, but I'd love to hear from a maritime expert.
After a month on the bottom, it's unlikely it would still be tumbling. One would expect it to have reached a resting place. If it's floating submerged... well, that would be a remarkable accidental balance of buoyancy, if possible at all.
Best guess... anomalous sound propagation. Although, it's surprising the detection ranges implied by that. Perhaps the ping detector is more sensitive than is expected for the "specified" detection ranges. Or, perhaps the sound was caught in a duct and thus was subject to a loss less than 1/r^2 spreading and 5db/km absorption.