Question: Was it ever revealed how the Psychlos invaded and conquered the Earth?
Answer: I believe that send an entire fleet of their mini-ships to earth by teleporting them (catching them off guard) and after settling on earth in the aftermath built a teleported on earth to survey the planet of its resources (unfortunately they couldn't find the gold which extremely valuable to them which was in Fort Knox, vaults and the Federal Reserve) and find an intelligent species to enslave to mine it.
Question: Why did the Psychlos subject Jonnie to that learning machine? Surely a race as intelligent as the Psychlos must have known that there would be unforeseen risks of subjecting a human to so much knowledge? Also, why not simply programme the learning machine to download a limited amount of information into his mind?
Chosen answer: To see if the man-animals could be trained to perform tasks more complex than simple mining. The machine wasn't programmed to restrict the information it gave him because, smart as the Psychlos are, they're also very arrogant. It simply didn't occur to them that a species as inferior as humans could possibly become that intelligent.
Question: How did the Psychlos originally find Earth?
Question: Is Jonnie correct in his assumption that there are other Psychlo colonies out there?
Chosen answer: Between the book and the hideous box office take, no he is not correct. In the book, Psychlo got turned into a sun and all the other colonies tried to teleport their ore shipments home and were wiped out by the resulting explosion.
Answer: If there were other colonies out there, they wouldn't love as much as their home planet just like earth. Besides even if there was, they've been mostly defenseless without their fleet on the homeworld and will already be on the verge of extinction.
Question: How could one nuclear weapon destroy the Psychlos' home planet?
Chosen answer: It's actually very similar to what scientists of the time thought would happen on Earth if a nuclear bomb was ever detonated. They were convinced that the entire atmosphere, being made up of several combustible gasses, would erupt in flames killing everyone on the planet. On Psychlos, their atmosphere reacted with the radiation released by the bomb. It's the reason the Psychlos could not enter the irradiated areas of Earth where the last groups of free humans lived: their breathing apparatuses would explode.
Question: Who's that weird little holographic alien Jonnie sees next to the learning machine?
Chosen answer: It's a Chinko. A subject race of the Psychlos. They were scholars and historians until the Psychlos got tired of them and wiped them out.
Question: Why are certain areas of the Earth irradiated? What happened?
Chosen answer: It is residual radiation from atomic weapons used in an attempt to fight off the Psychlos.
Question: What exactly are 'gas drones'?
Chosen answer: Huge robotic ships programmed to fly on a set course and drop poison gas.
Chosen answer: They teleported a gas drone into Earth's atmosphere. The drone flew around the entire world and dropped poison gas which killed Earth's population.
Grumpy Scot