question about main vs load
Its perfectly normal for the main breaker to be smaller than the sum of all the breakers.  This is fine, because its rare that you would turn on your electric stove, dishwasher, hot tub, central air, electric furnace, dryer, fridge, and arc welder all at the same time.

Now, its quite possible that the electric service was undersized for today's use, but that should have tripped the main breaker.  A fire could be caused by a lose connection, which creates resistance, which generates heat.  Or a bad breaker that failed to properly trip.  Or undersized wires, where someone put a larger breaker than the wires were meant for - the modern equivalent of replacing a fuse with a penny.

Aluminum wiring has also been known to cause fires if it wasn't properly installed and maintained.  It can be used safely, but is harder to do right than copper wire.