ventus45...
Could it be, that both recorders separated from their mounts, or more likely, that their mounts separated from the local strucure, such that the recorders sank to the ocean floor as small dense packages, and may by now have sunk into the mud ?
Yes indeed. If we accept BEA's view of the likley impact sequence, that outcome is certainly in the area of "possible" as opposed to "unlikely."
If so, what chance of locating them if they are not visible ?
Depends very much on whether they are lying (buried) within, or very close to, the rest of the debirs field.
Given the forces involved, what real confidence is there that the recorders were able to remain whole anyway ?
I am optimistic (as are others I have spoken with) -- but certainly not confident.
Could we be in a situation, where they are not able to be found, and even if they are, that they are themselves so disrupted by impact forces and subsequent pressure effects that no useful data is recoverable ?
Yes of course.