How does Conrad convey a tone of ambiguity?

Joseph Conrad utilizes ambiguity throughout Heart of Darkness through several methods. The first, and perhaps most significant, method of portraying ambiguity is Conrad's use of a frame narrative. By filtering the tale through several first person narrators (there is the narrator of the novel itself and Charles Marlow, who speaks to the narrator in telling him the actual story of the novella), there becomes an increasing sense of untrustworthiness. If either the narrator or Marlow were to be lying or exaggerating, the reader could not know, and thus there is a sense of ambiguity in the level of trust.
Second, Marlow's feelings towards Kurtz, the natives, and the idea of colonization itself seem to be continuously fluctuating. Marlow seems to admire Kurtz while also being disgusted with his inhumanity. He is excited by the idea of exploring the unknown terrain but wary of the greed-based motives behind colonization. In a further contradiction, Marlow appears to be critical of the primitive nature of the natives while simultaneously respecting their power.
By setting Marlow up as a morally ambiguous flip-flopper, and by widening the possibility of a lack of credibility, Heart of Darkness becomes a work of great ambiguity. What Marlow believes and what Conrad intends to say becomes difficult to decipher.