This is an interesting question and you have made many pertinent points, but it remains unclear to me why a move from listening to silent reading creates selective pressure for styles that can be received and understood quickly. If that is an advantage in silent reading, why less so for the same words spoken? After all, listening seems to be burdened with a few additional barriers to comprehension, such as in disambiguating homophones and the inability to skip backwards and re-hear what was just said.

The preference for brevity in telegraphy and newspapers does not strike me as evidence for the above proposition (and might be regarded as examples of the phenomenon to be explained, rather than part of its explanation.) In particular, telegraphy is actually aurally-received communication! In the case of newspapers, an alternative hypothesis lies in there having been clear pressure to compress the message into few column-inches.

You have presented evidence that writers today tend to use longer sentences while speaking than when writing, which (if it holds generally) is consistent with the view that brevity is more valuable in silent reading, but it does not, by itself, establish that as a fact, and it is also consistent with alternative hypotheses, such as speech being produced in real time, without much time for optimization.

One could say much the same about the Flesh-Kincaid readability scores, unless there is evidence that this holds less strongly (if at all) for the spoken word (the observation of writers being more loquacious when speaking is not sufficient to establish that: we would need evidence that long spoken sentences are easier to understand than the same thought spoken as one or more short sentences, and then we would want to understand why this is not the case when reading.)

The problem with facts speaking for themselves is that they rarely do; on the contrary, they are frequently cryptic. That being so, perhaps the next-best thing is that people should be familiar with the dark arts of rhetoric, so that they are better positioned to recognize and respond to their use. The measure of credibility is only saying things that stand up to scrutiny.

With a steel ball, air resistance and the Magnus effect were apparently not an issue. Now I'm curious as to how it goes with either a ping-pong ball, or a golf ball and a longer drop to the floor.

I get the feeling that a one-dimensional view of directed/purposeful behavior (weak -> strong) is constraining the discussion. I do not think we can take as obvious the premise that biologically-evolved intelligence is more strongly directed than other organisms, which, in many different ways, are all strongly directed to reproduction, but it seems there are more dimensions to intelligent purposeful behavior. There are some pathological forms of directedness, such as obsessive behavior and drug addiction, but they have not come to dominate in biological intelligence. We generally pursue multiple goals which cannot all be maximized together, and our goals are often satisfied with some mix of less-than-maximal outcomes.

So, even if coherence does imply a force for goal-directed behavior, we have some empirical evidence that it is not necessarily directed towards pathological versions - unless, of course, we have just not been around long enough to see that ultimately, it does!