[eno_]

Interesting, but... Would not a commercial airliner off Long Island be "lit" by several radars?

It is surely a hack to take a truck-mounted SAM and fire it from a beach or a boat, but it is hardly impossible. The point is that limiting the possiblities to man-portable missiles isn't neccessary. It also contradicts some of the eyewitnesses aloft who claim to have seen a bright light, like the exhaust plume of a missile.

[Rokke]

"Would not a commercial airliner off Long Island be "lit" by several radars?"

Yes, but a surface to air missile doesn't guide on any radar energy. The target tracking radar that guides a missile like an SA-6 provides an incredibly powerful and very narrow beam of radar energy with specific frequency, pulse width and modulation. The missile only "sees" that radar illumination.
I agree that it is incredibly unlikely that anything was launched from land the night TWA800 went down. No SAM system is portable enough and descrete enough for that to happen without thousands of people on Long Island being aware of it.