Complex sentence: David discovers through research that Stephen Falken, an early artificial intelligence researcher was the system designer who had (supposedly) died several years earlier, and guesses correctly that Joshua, the name of his son who died as a child, was the backdoor password.
Split into simpler sentences: David discovers through research that Stephen Falken, an early artificial intelligence researcher thought to have died several years earlier, was the system designer. David correctly guesses that Joshua, the name of Falken's son who died as a child, is the backdoor password.