I absolutely loved this story, which may be one of the great classics of this series. You've got most of the story dead to rights, but one thing needs to be noted: The mysterious stranger is one of the two lost explorers who created the antidote to the insanity virus. Upon discovering their lost treehouse, Spirou believes the skeletal corpse he finds in the bed is the last survivor, having buried his friend and forsaking a tomb for himself.
Said stranger was immunized to the insanity virus, which is why he wasn't a ranting violent maniac, although by that last page it's clear he went somewhat loopy on his own accord, treating the skeletal corpse as his roommate, and stating that his late friend Gunter still won't speak to him after all this time.
The skeleton himself is Lord Malabar, apparantly the last ruler of a dead nation that Genghis Kahn conquered, but as they were all pacifists, refused to fight for him. So he decided to punish them by sending them all to that lost valley, where the insanity virus took hold of them and caused them to fight to the death.
And I think people confused by that "Call of Hope" thing should know, that "Houba" was the call of the Marsupilami, a cryptozoological wonder that used to accompany Spirou and Fantasio back when they were written and drawn by Andre Franquin. Honestly, he deserves a series of scans of his own.