(4) Logistics...good idea! The HEMTT is awesome. But in addition to all of the above problems, I think you'd see guys taking HEMTTs for themselves, to go looking for their families. I think you'd see desertions; certainly you'd se significant breaks in the logistics chain at various points as areas became overrun and began looking out for themselves. A Cav unit defending Atlanta against several million Zs might be written off.
Not only that but what about the people in the factories and gas stations and warehouses?
What good is a supply line if the first point in the line, the manufacturer, can no longer operate?
You can have all the trucks in the world but if you don't have oil coming into the country to be turned into gas you've got nothing when the oil reserves in the country go out.
Then of course can you get everyone in uniform? How many people for the national guard or the reserves are actually going to show up for duty?
For the record, I wholeheartedly agree with you about wondering of the fate of the military. We see a lot of stopped tanks and vehicles, but not a whole helluva lot of dead soldiers.
We've seen some wandering around as Zombies but I do agree there is a chance they retreated but I think it more likely that things broke down to far and they just took off.