[Mike L]

You're right, I'm hung up on what appears to be a requirement (per the casebook) that the receiver must be downed in order for forward progress to apply.
And, admittedly, I'm also somewhat hung up by a clear ruling in NCAA that the play in question does not result in a TD and my not being able to find any substantial difference in the applicable rules between the two. The only difference being in NFHS the ball must be possessed in the EZ (but in this case the catch must also be completed) and the NCAA says the receiver must catch the ball in the EZ. So from what I can see, the problem remains no clear interp from NFHS since the one given has the pesky "is downed" component, while NCAA has a definite interp.