“A MANPAD doesn’t have a warhead large enough to bring down a 747 in a catastrophic manner.”
Most MANPAD warheads have ~1 - >3 lbs of high-explosives in the warhead. Exactly how much would it take to detonate the fuel in a jumbo jet fuel tank?
And they are designed to home in on engine exhaust which in the case of a 747 is out on the wings. A near miss to an engine could result in the MANPAD hitting the center fuel tank area at which point the missile's contact fuze would detonate, or a timed/proximity fuze might have detonated.
Regardless of whether a MANPAD exploded via direct contact, or nearby via timed/proximity fuze, there would be evidence of the aircraft's skin being penetrated from outside to inside.
The "official" explanation is that fuel in the partially filled center tank was heated on the ground by nearby air-conditioning equipment. Once in the air, the relatively small amount of heated fuel in the center tank sloshed around and created a warm fuel vapor inside the tank. Some sort of electrical discharge then ignited the fuel air mix.
Because the main "official" theory is a fuel-air explosion in the center tank, A hot piece of missile shrapnel, or burning explosive could have penetrated into the tank and ignited the fuel-air mix but again, there would be evidence in the form of penetration(s) from the outside inward to the fuel tank. I've never heard of any such evidence.