Wouldn't work unless all the planes were landing near simultaneously. The Brits suggested 10 airliners, but after the first one or at most two detonated over a city you can be assured that no more incomings would be allowed over cities.
You're right, but I don't think that would have to be the case. I'm guessing they'd probably pick a time right when the first set of planes was to be over a city, and make that 'go time' for all parties. It would be safest for them, and more terrifying for us. Some planes would be over cities, some over land, some over water. We'd have the full range of crash site dramas unfolding; desperate searches at sea, urban sites with chunks of airliner spread around, rural hills with emergency vehicles swarming around. A true disaster.