1. In the book they mobility kill AFV's by getting in the way till the ground up bits causes something to break, (thrown track in the case of an abrams).
2. I'm not remotely suggesting that they stand there till the zombies get them as such. That's not really realistic unless it's an objective that has to be defended. Which was sotr of what the first battle of the book was about, the zombies had started moving out of NY trying to break quarantine and they had to be stopped before they dispersed into the countryside or they'd be even harder to deal with. Even then most got away apparently. They just couldn't stop the zombie horde with what they had.
3. All of that prep sounds nice, if you can do it. The point about he WWZ scenario is that they couldn't. Again going from memory of the past discussion the timeline starts with the initial disease in NY with no one knowing it was a zombie plague, they try and fail to contain it and are slow to realise it is a zombie plague. By the time they do they've got outbreaks in every city, NY is overrun and it's gone global. Martial law has been declared everywhere with troops backing up the police and more troops around every military stockpile and base to defend against rouge zombie groups. But NY seems contained, the zombies aren't leaving so they put a small quarantine force in place, (few thousand troops, a company of tanks), to maintain it against the odd small zombie group and pour the rest of their resources on trying to piss on all the bonfires they have to deal with, (and be fair, in a modern western military on home soil there's going to be a very strong tendency to not abandon anyone by the higher ups, they're US/UK/French/German/Whoever citizens). When the zombies suddenly decided en mass to move out of the city all the brass could commit was a few planes.
The discussion focused on that battle so my knowledge of what happened afterwards is very hazy, but from what i do know it sounds like roughly the same pattern kept repeating with a severely overstressed military, (with an equally overstressed logistics system), trying to be strong everywhere kept getting overrun in small groups because no one group had enough men and heavy ordnance to make it work. They basically suffered a severe case of defeat in detail. At that point they have few cities, no real manufacturing, (or oil refining), pipeline and millions of refugees to look after with tens to hundreds of thousands of swarms of zombies numbering anything from a few tens of thousands to a million or so spread all over the US and wandering in every direction.
Sure they can do that. If that have enough guns and shells and can concentrate adequately, the US only has so many guns though, (and much of the rest of the world proportionally is even worse off), and the point about the WWZ scenario was that they basically couldn't/didn't concentrate in the early stages trying to hold onto every city and town and protect every part of the US at the same time. And i don't think thats unrealistic. However practical abandoning some area to focus on some area's first then going back and clearing the abandoned area's afterwards may be i just can't see anyone doing it without a lot of messes when citizens of their own country are involved.