Doing something like that on the Falcon 9 would only take two Orion abort motors (which have 500,000 lbs thrust each and a 5.5 second burn). It's pointless on the Falcon, but they're not blowing a billion dollars on facilities. Air-starting the first stage shouldn't be shocking because we've been air-starting second and third stages, in flight, since the 1940's.
Wrong, the structure can not take the propellants much less thrust of the abort motors. And solids would be unworkable since they can not be controlled.
Also, booster engines are not the same as upperstage engines and many are not air startable.
All your examples of horizontally integrated vehicles, not one of them are fueled until vertical.
This idea is just plain stupid.