So, if it's true that these KH-12 spy satellites are basically modified versions the Hubble space telescope, would it be reasonable to assume that Webb will get copied in a similar way for spy satellites?
Yes, the spy satellites were first by a long shot. The KH-11 (2.4m mirror, CCD focal plane) was first launched in 1976. Hubble (2.4m mirror, CCD focal plane) was launched in 1990.
The space shuttle was funded and designed to meet Department of Defense specifications. Its cargo bay designed to ferry satellites up and to potentially return them back to earth.
Keyhole satellites were seriously cool. Photos were taken on film, and the film reels were jettisoned over the ocean. C-130 transports would capture the payloads as they parachuted back to earth:
Doubtful, I think Webb only sees in the infrared. Looking at the earth would blind it. This is why it'll be a million miles away protected by a fancy heat shield.
Wouldn't that depend on how much dynamic range it has? Webb is intended to look at very dim signals in space so Earth would blind it, but if a spy-Webb were intended to track rocket or jet engines, things quite hotter than the rest of earth, perhaps it might be capable of that?
Unlikely. You have the origin mixed up. The spy satellites came first, and then Hubble.