No it wouldn't. Around here I know of many intersections that only display the walk signal for a second or two before going into the countdown, such a short time that you can miss it if you briefly glance away. I pay attention to crosswalk signals and spent months training my dog to do the same so that he sits when we arrive at a crosswalk. When the city is setting up the lights so that they change in the time it takes to walk a single step then your interpretation isn't reasonable.

Some intersections I cross in Chicago have the walking man for literally one to two seconds before it starts the blinking hand or countdown (typically in the 10-15 sec range). This is barely enough time to enter the intersection, even if the individual is paying complete attention. Given this, I'd never even considered that it could be a fineable offense to begin crossing when the countdown was happening.

That is a joke -- people walk at different speeds. Walking in carland is enough punishment. The worst are burbias where the walk signal never comes on if you don't press a button.

Not if you take the walk signal to mean "it's safe to cross right now," which is not the case if you're on the curb and there's one second left.

You also clearly haven't spent much time in L.A...many of these signs flash the walk signal for literally 2 or 3 seconds before beginning the 20 second red countdown.

No other place treats it that way (and most police departments have better things to do than to target pedestrians).