John R, the grandfather paradox is a true physical paradox, regardless of philosophy.
You write, " ... leads to an argument about form which treats light velocity as the limit at which 'time stops' rather than acceleration stops. Time continues for the wavefront."
You're forgetting that in relativity neither time nor space, independently, are physically real. When we speak of motion, we have to ask -- relative to what? Time doesn't stop -- for an observer whose motion is less than light speed -- time stops for the particles of light (photons) to whom the observer is always at relative rest.
You continue, "There is no paradox of grampa getting old where his gravitational reference is progressively lagging behind times' extention, in relation to acceleration to light velocity. We need not know why nature settles on that finite velocity to hypothesize that, consistent with Lorentz, the higher relative velocity, the slower the rate of acceleration which propagates as velocity. At greater than light velocity, acceleration rate would diminish faster than propagation of velocity."
Again, though, relative to what? The grandfather that the paradox refers to may or may not be old -- that's irrelevant. It also doesn't have anything to do with gravity or acceleration. The paradox exists because it has no classical analog where gravity and acceleration would apply. The paradox hypothesizes a quantum event (assassinating one's grandfather before one is born) which violates causality (a distant cause of one's birth, which after the event has occurred, could not have happened).
Classical physics has no way out of the paradox. As I suggested previously, quantum physics might have a resolution, if Everett's many worlds interpretation is true. Hawking, in fact, is purported to have said that many-worlds is "trivially true." I agree with that statement, for the reason:
Neither space nor time having any reality independent of spacetime, a quantum event is a singularity in spacetime -- which makes Hawking's purported statement easy to understand, because the research for which he is famous deals with events on the boundary of classical black hole physics and quantum mechanical formalism. If the black hole singularity is the origin (via Hawking radiation) of many particle trajectories, it is also the origin of many spacetime histories.
Early in his research, Hawking believed that black holes destroy information. A few years ago, he reversed himself in favor of information conservation; i.e., instead of allowing entropy and infinite density to make information entering a black hole irrecoverable, Hawking radiation makes more sense when what goes in, comes back out.
For the grandfather paradox, I think that this would mean a one-way memory of events for the observer. If one asks the question, "Did I go back in time before I was born, and kill my grandfather?" the answer has to be "No." If it weren't, one would not be around to ask the question. This also means, however, that travel into the past of this (or any) spacetime history is not possible while travel into the future of this (and any) spacetime trajectory remains trivial.
Symmetry, in other words, is preserved between past and future events, and not between past and present or present and future. This is tantamount to saying that all physics is local, as Einstein averred. The local past and future is identical to the global past and future (classical physics) of a discrete spacetime history (quantum physics).
