From: Utlima Thule
It's tough playing the Western Allies, and one reason is the sheer rigidity of the amphib process. The WA can spend weeks prepping for an invasion, only to discover at the last moment that that a German division has appeared at the landing site. Which means either landing against a strongly fortified position, or waiting several more weeks while retargeting the invasion.
I think the real-life Allies would have had a little more flexibility. Is it possible to upgrade the amphib planning process so that shifting the landing site by one or two hexes only incurs, say, a 15 percent reduction in prep points? The Allies would still pay a penalty for changing the invasion site, but without having to start all over again.
I think you are missing the impact of rule 16.7.3 in particular:
During an amphibious invasion attack against a non-port hex, the defending units may only use the
CV value of the largest unit in the hex, and if that unit is a larger than a regiment or brigade, it will only use
1/3 of that unit’s CV value. All units will participate in the battle, but only the one unit will count in the
after battle CV value that determines the winner of the battle.
So if a Pzr division turns up at the last moment, a well prepared invasion will take more losses but it doesn't affect the end chance of successfully getting onto the land.
The key here, and I think this is shown in almost all the AARs is that a big critical invasion has to be well prepared. High prep values, a well chosen load out for the assault divisions and as much airpower as you can manage. Do that and last minute axis moves are irrelevant to your chances, in fact I'd like the Germans to put a Pzr division onto an invasion site, it'll take horrendous disruption losses from air power and naval guns.
A poorly prepared invasion is indeed vulnerable but I'd suggest the lesson of WiTW is that poorly prepared invasions tend to fail in any case.