PAllen said:
rigidity such that it can retain its shape under compressive forece
"Retain its shape" in the sense that, while supported, it retains a rectangular rod shape, yes, but it can still have significantly reduced thickness as compared to its unstressed thickness.
PAllen said:
such resistance force is able to overcome the gravity after this delay.
The resistance force can't "overcome" gravity, because, by hypothesis, the rod is unsupported in the hole rest frame while all this is happening. There will be an induced vibration in the rod due to the internal forces as shear waves pass through the rod, but the net effect will still be for the rod as a whole to fall as long as it is not supported, and the downward acceleration of its center of mass will be the "acceleration due to gravity", even if individual particles of the rod have different accelerations due to internal forces. The rod can't magically suspend itself.
By hypothesis, the acceleration is large enough for the length contracted rod to fall through the hole in the hole rest frame. In that frame, the downward acceleration is applied at the same instant to all parts of the rod. That is the hypothesis I am working with. Under that hypothesis, while the rod might indeed "snap" to a configuration that looks rigid in some frame, it will still fall through the hole.
In the frame in which the rod is at rest prior to starting to fall, the support is removed first from the front end of the rod. Note that in this frame, the downward acceleration on unsupported parts of the rod is increased by a factor of ##\gamma^2##. This means that it is quite possible in this frame for the shear on the rod to happen too fast for the rod's internal forces to compensate, so that the rod bends in this frame, even if in the hole rest frame the rod's motion looks rigid.
PAllen said:
It is an independent local reponse to each force application event - it doesn’t have travel, e.g. from an end of the rod; it just has a local communication delay
In the hole rest frame, this is true, since there is no time delay horizontally across the rod in this frame. But in this frame, as above, the "local communication" can't possibly prevent the rod from falling, because all of it is unsupported at the same time.
In the rod rest frame (the one in which the rod is at rest prior to starting to fall), the support is being removed horizontally along the rod at almost the speed of light, so each part of the rod is losing its support at about the same time as it is receiving the information that it needs to pull back on the part of the rod just ahead of it. So "local communication" is not sufficient to keep the rod from falling (or more precisely in this frame, bending) in this frame either.