That's impossible to quantify, and it isn't linear. For example, let's say Wiggins is that dude who if he wasn't fat, but just moped around, could legitimately hit a 32" countermovement (don't underestimate how impressive that is). The nature of gains is that he could be a rather casual gymgoer with a solid program, and probably hit 36"-37" without too much time or grueling intensity. Hell, probably in 6 months. That's how quick those initial gains come.
Going from 37" to 42", though? That's insane. That last five inches requires literally several dozen times the effort to make than it does to make the first five. It gets to a point where these physical freaks at the top are separating themselves by a matter of inches, or even centimeters, but 4 centimeters up where they're at represents a greater number of standard deviations than 4" in the bracket 10" below.
You don't get a vertical like that at 6'8" without busting your ass. They aren't common by any measure of the word, but within the context of the global population, I'm sure there's a reservoir of tall guys who can jump 30"+ without lifting a finger. 40"? Extremely unlikely. Someone like that is probably the type of talent who hops onto a D1 track and field program and is medaling at the Olympics 3 years later when everyone else has been training the sport for like 10+ years.
Certainly. Agreed on points #1 and #3. Keep in mind, though, with #2, that even submaximal repetitions are a function of your 1RM. So a guy with a 40" vert can jump at 90% of his max and still hit 36". Meanwhile, a guy with a 36" vertical is screwed in that equation. Of course, power-endurance isn't necessarily equal, and factors into an athlete's ability to reproduce these efforts at a certain intensity over the course of a game.