Am I reading this right? Are the hugely knowledgeable (and most welcome) posters saying that the subsurface pinger signal may be subject to range extending effects just as HF radio waves are bounced/skipped off the ionosphere?
Basically yes. Differences in the properties of a material, be it something as simple as a glass fiber or as complex as the atmosphere or the ocean can cause signals (acoustic, electromagnetic, or optical (a special case of EM)) to be bend. If the conditions are correct, the bending tends to refocus the energy of the wave in a channel so that the loss in energy becomes linear in distance rather than the normal quadratic (1/r^2). For those old enough to remember early microwave (not oven), the medium becomes a natural wave guide.
Physics may not find MH370 but the laws it provides still do provide rational explanations for all manner of natural phenomenon which have come up as a result. Sadly the "laws" of human nature don't.