You make good points.
First as why a terrorist would not take credit?
Remember Libyan agents placed a bomb aboard a Pan Am flight, though neither Libya or any terror organization claimed credit. Qaddafi Duck wanted to settle a score with the U.S. for the raid on Tripoli. The Iranians felt they had a score to settle due to the Vincennes incident, and Saddam certainly nursed a grudge going back to th first Gulf War. (Note, he tried to kill President Bush in 1993.) So this point is not convincing.
Why weren't there more such attacks, why did they stop?
If it was the Iranians they might have considered the score settled, one for one. If it was Saddam, he may not have wanted to press his luck, having avoided detection once. If it came out that someone was taking potshots at U.S. aviation, there would have been an outcry for retaliation and prevention comparable to the reaction to 9/11. With elections looming in 1996, Clinton would have felt compelled to act.
The conclusion of the investion is far too convienent for Clinton and his cronies, for whom absolutley everything was viewed through the prism of national politics, for whom there is no principle except getting elected