Your diagram misses the most essential feature. The ends of the main trusses were bolted to the core and the outer wall using two 5/8” bolts on each side. Those bolts are what failed (at least were the main cause of the general failure), owing to the stresses of the floors deforming, and the overloading due to the junk falling onto the floors from collapsing floors above. As the trusses sagged, they pulled against the bolts and the seats that the trusses were bolted to. Imagine a long wooden plank that is placed between two buildings over an alley, such that the plank just makes it across, with an inch of it on either ledge. As you put more weight on the plank, it will deform, curving downwards. We all know that a curve between two points is longer than a straight line. When that difference hits two inches, the curved plank can no longer even stretch over the gap. That is what happened to the trusses, except hat the ends were pinned by those bolts to small steel ledges welded to the upright beams. The more deformed they got, the harder they pulled - not vertically, but horizontally, off those small seats, as the bolts (and some of the seats) failed. There are plenty of photos of these failed connections.
Here is a photo of a floor under construction (there is no concrete on it yet - the concrete would have just covered the parts of the wire sticking out of the floor):
The wider seats are where the trusses were bolted, the narrower ones for the dampers (which serve to dampen normal vibration - they do not tie the trusses to the walls). The small seat above and between the wide ones is to tie horizontal stabilizing elements.
Here is a photo that shows the effect of the sagging trusses pulling on the outer walls:
The steel beams on the outside of the structure were extremely resistant to vertical forces - but not to horizontal ones. They needed the horizontal trusses to keep them from bowing inward or outward. The best example of how that works is the old stand on a can thing - you can stand, if you are careful, on a regular aluminium soda can, and it will support your weight, so long as you keep the forced running straight down through the metal. Any slight deformation laterally, and the can fails suddenly. The same is true of jacking up a car - on flat ground it it no problem, but on a hill you have to be very careful to make sure that the jack is perfectly vertical. Even on flat ground, if anything shifts the car so that the jack is no longer vertical, it comes crashing down. The outer walls of the WTC were like these things - very strong vertically, but only vertically. When they deviated from enough from vertical, they could no longer direct the force of the weight above them downward, and enough was directed horizontally that they failed.
psikey, you have no concept of how structures work. There was a tower, a radio mast, being put up. They were lifting a set of antennas using a bolt that was too small. That bolt failed, which overloaded a second bolt, which also failed, dropping the antenna array. That structure fell onto a guy wire, which failed. That led to the guy wires on the other side of the tower to pull too hard, deforming the tower, which swayed once, then back in the opposite direction, then crumpled. The failure of a 2,000 foot tower was due to the failure of one bolt.
The WTC was more robust than such a tower, but the same rules apply. The failure of a 1400 foot building was due to the failures of hundreds, thousands, of small structural elements. There is only so much a structure can be weakened before it fails.
All your talk of how much heat it takes to heat how much steel is completely irrelevant.
How much heat does it take to weaken a chain made of of one billion links of 100g each to the point of failure? That’s a hundred thousand tons of steel. Could you break the chain with a blowtorch? With one pair of bolt cutters? About a minute with a hacksaw?