] Lucy's plan seems clear enough. Having caught Edward, she was hoping his mother would settle some money on him (as the Dashwood's were hoping earlier, in fact). Failing that, she seems prepared to wait it out for Mrs Ferrars' death.

Two things work in Lucy's favour. One is an understanding of Edward's character. She knows he won't dump her or abandon her having made a promise to her. The other thing is that she is likely very conceited. She is convinced that she is "good enough" to break through and weasel her way into the Ferrars family. She is confident in her own ability to make it happen somehow. So at this point of the novel she's working her way towards the Ferrars family: if Edward won't do the deed of revealing the engagement she will somehow!

] Anyway, while Lucy sits around praying for a freak outbreak of plague to wipe out her enemies, what is Edward's plan?

I don't think Edward has any plan at all, frankly. He probably had no clue that with more time his mother would just become more adamant that he make his way in society and marry well. He probably also had no clue that with more time Lucy would just become more and more inferior in his eyes.