What does Liesl propose to Dunstan and how do they resolve this Proposition?

I think she means solving it with some heavy Jungian therapy. Like some Faustian bargain, Liesl offers Dunstan a glimpse into his grandest imagination and the darkest recesses of his soul in exchange for his presence and his writing skills. Liesl is incredibly introspective but firm with Dunstan. Like Mephistopheles, Liesl guides him through the dark baggage of his soul, the baggage that he previously refused to open. What Dunstan finds is certainly frightening, but is also liberating. In effect, what she refuses to do is to play his game of intellectual dodges and diversions. Instead, she aims for the truth: we all have desires and must work to realize them.