[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.

[eno_]

You are ascribing fantastic properties to the SAM-6 radar. The missile has a range of over 40 miles. There is no technology that would radar illuminate a plane in a narrow beam like a searchlight over that great a distance. The ground radar is quite normal, and covers a large segment of the sky.