I agree with most everything you have. However a small quibble with one part of your explanation. At least as how I understand the "crackback" rule.
The ball leaving the tackle box does not restrict all offensive players from blocking below the waist toward the ball. The ball leaving the tackle box does restrict them from making a block below the waist toward their own end line (a "peel back block"). And it also restricts linemen to blocking below the waist only from 10 to 2.
But the ability to "crackback" is determined by whether the player has been outside the tackle box or was in motion at the snap.
In today's "inclusive" environment, I was a tiny bit too inclusive. Yes, a lineman within the tackle at the snap, and
who has not left the TB, may BBW toward the ball from the side or front, until the ball leaves the TB. After the ball leaves the TB, and such lineman has not not been outside the TB, such lineman may BBW from the front, toward the ball, within the tackle box. But, 9-1-6-a-2 clearly tells us that a Team A player - any Team A player - outside the tackle at the snap,
or who is outside the tackle box any time after the snap, may not BBW toward the ball.
So, how would that happen (legally)?
1) 1/10, A-30. Lineman A66, immediately to the left of the snapper at the snap, has retreated to the A-28 and is unengaged, when he see B77 escape from the snapper's block and is moving directly toward BC A11, who is in a passing posture. At the same time, A11 has released a pass and the ball has left the TB when A66 dives to his right, toward B77, contacting B77 at the knees, and within the "10-2" region. The contact occurred at the A-29, immediately behind the spot where the ball was snapped.
Ruling: Legal BBW, toward the ball, after the ball has left the TB, because A66 never left the TB. And quite plausible, too.
2) 1/10, A-30. Lineman A66, immediately to the left of the snapper at the snap, is blocking B99 to the ground at the A-29 and approximately 2 yards to the left of the spot where the ball was snapped. BC A11 has carried the ball 8 yards to the left of the spot where the ball was snapped when A66 moves to his right, and blocks B77 directly from the front, at B77's knees, at the A-29 1/2, and one yard to the left of the the spot where the ball was snapped.
Ruling: Legal BBW, toward the ball, after the ball has left the TB, because A66 never left the TB. This is plausible, although probably very, very unusual.
3) 1/10, A-30. Lineman A88, 4 1/2 yards to the left of the snapper at the snap, moves directly back to the A-28, anticipating a quick pass from A11. A11 muffs the snap, recovers the ball, and moves to his left. When A11 has moved 8 yards to the left of the spot where the ball was snapped, A88 (having remained in place to this point) moves to his right, and, at the A-28, dives at B77, contacting B77 directly from the front at the knees at the A-29, at a spot 4 yards to the left of the spot where the ball was snapped.
Ruling: Legal BBW toward the ball, because A88 never left the TB.
So, what would an illegal BBW toward the ball by a lineman look like?
4) 1/10, A-30. Lineman A66, immediately to the left of the snapper at the snap, moves to his left at the A-29, leading blocking for BC A11. A66 has moved to a point 8 yards from the spot where the ball was snapped, when he looks over his right shoulder and sees B77 chasing and closing rapidly on A11. A66 reverses his movement, and approaches B77 directly from the front, and dives at B77, contacting B77 at the knees, at the A-29.
Ruling: Illegal BBW by A66, as he had been outside the tackle box, and the block was toward the ball.
5) 1/10, A-30. Lineman A88, 4 1/2 yards to the left of the snapper at the snap, moves two yards directly to his left at the snap and back to the A-28, anticipating a quick pass from A11. A11 muffs the snap, recovers the ball, and moves to his left. When A11 is 8 yards left of the spot where the ball was snapped, A88 moves back to the right and, at the A-28, dives at B77, contacting B77 directly from the front at the knees at the A-29, at a spot 4 yards to the left of the spot where the ball was snapped.
Ruling: Illegal BBW, because A88 had left the TB.
So, yes, under a very restricted set of circumstances, a BBW toward the ball after the ball has left the TB, can be legal.