What is your source for your statement: "that huge fireball exploded no higher (and quite possibly much lower) than about 7500 feet." I just read some stuff that says the CIA animation has the last recorded altitude at 13,800 before the sudden (impossible) huge climb. I just read an eyewitness account from two airline pilots who saw the whole thing who initially radioed in to ATC that the huge fireball explosion occurred "around 16,000 feet". So what's your source for 7,500 feet. That number appears to be a statistical anomaly in TWA 800 literature.
DJ MacWoW included a link to an inaccurate news article about witnesses Faret & Wendell in his #883 and I provided him with their own report in my #885.in which they indicated that their own flight altitude was 8500 feet when the "huge red-orange" fireball ball exploded below them, that when they flew over to the smoke cloud it left - "we were at 7700 feet and were at the top edge of the cloud. The cloud center was at 7500 feet" - and there was nothing unusual in the sky above that altitude.
The NTSB's Exhibit 4-A includes airborne witnesses' altitude estimates of the huge fireball ranging from about 4000 feet to about 8000 feet.
As I recall there was also a report that a US satellite had detected the huge fireball explosion about a mile above the surface.