How does Marlow describe the Congo River and its environs? How does he describe his interaction with the River? What illusion does the River promote? What insight does it provide, at least so far as Marlwo is concerned?

The illusion is the fact that the river is seen as the gateway to Africa. The journey upriver provides Marlowe with the insight that Africa wants to keep the Europeans from entering by keeping their progress as slow and difficult as possible, whereas the trip back downriver is a quick exit from a place the Europeans are not welcome.