"My son is just at the point where he's gathering a list -- a long list -- of possible schools. This is just part of the information base. The hard part is knowing where a 15-year-old sophomore is going to end up! 
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Where he ends up is sort of the beginning, even though I fully agree with you that, at his age, it seems like an "end."
My impression is you are getting a pretty good grasp of things. Because I assumed you put in plenty of time and effort, I felt it important to post (I have pretty much stopped posting.)
What I hoped to communicate with the velocity comparison for the Trinity pitcher was how much change there can be (and usually needs to be) with many players from a point in time in HS into mid-point of a college career.
Having talked with a number of college coaches, and being the Father of one who coached in college, that, to me, it a major difference in how situations are viewed from the player/parents perspective as contrasted with college coaches (other than that top 3-5%)
No matter how hard we try as parents, it is problematic for us to understand how much better our son's need to be to compete once they get beyond HS. (again, leaving out that top tier of players who have challenges but perhaps different ones in many ways at top tier programs.)
Because they succeed or fail on the ability to project a player they see at age 16-17, some college coaches truly have skills in seeing that HS junior or senior and projecting where they can get with good coaching and a ton of hard work by the player. It is that gap, and all the hard work needed, which makes me cringe when a parent of a recruit posts and indicates their son will be getting playing time as a freshman in college.
Good luck to you and your son along the pathway. I know the end/beginning seems like a long way off. It isn't.