Well, considering how long it took to rebuild the World Trade Center (aka The Freedom Tower), I'd say that total rebuild of any one of those cities would be impossible. Even if you had every construction crew in the country with no political red tape to get in the way, it would be hard to rebuild even just half of Manhattan.
Politics. Hopefully all the useless politicians were killed in the first strike (although I doubt it. I see Blair survived)
http://www.historiccoventry.co.uk/blitz/recovery.php is how Coventy recovered from being flattened, during a war where there were still attacks. Hiroshima was rebuilt in about 5 years once they started.
The damage per city wasn't terrible - the statue of liberty was knocked over, but people fairly close to the epicentre survived on the metro and in tunnels, fire engines were still drivable, etc.
As for worldwide deaths, Lets assume that there were 36 initial attacks (3 dozen). Say an average . We only saw the poor evacuation of the US cities (not using the other half of motorways), there's no suggestion that other countries also waited to evacuate, but even then people managed to get out, drive from NY to Washington, and pull up at the whitehouse without much of an issue.
Evacuation of a large portion of the cities seems fair enough -- Will Smith's neighbours were all bugging out at the start of the day, only people who went into work and stayed there until the evening of the attack got caught in the traffic.
Washington's population is under a million. Berlin 3, Paris a similar amount. Karachi, NY, LA and London are higher, but Jerusalem, Vladivostok, Athens etc are lower. Lets say the first wave hit 36 cities with an average of 2 million people each, half of whom managed to evacuate. That would be 36 million dead.
After that, there would be limited mass casualties - everyone would evacuate, and this was confirmed with the second phase of attacks. There were a further 72 cities destroyed, (although not my own home city of Manchester - not only did they blast Birmingham, which would be an improvement, but they then targeted Liverpool! Scotland, Wales, Ireland, Portugal, Iceland, Spain, Madagascar, Peru, etc were completely untouched)
I can't see more than 50 million dead, or under 1 in 100 people.
The centre of some key "landmark" cities will have been rebuilt for morale purposes, London, Washington, etc. Others may have been put on the "to-do" pile, but there's a lot of people available to rebuild, and 20 years is plenty of time, alien tech or no alien tech.
