...can she simply not predict

(1) how clumsy Darcy will be in letting her know he is in her power
(2) that Darcy's role in the Bingley/Jane split-up will come to light just before Darcy chooses to make his feelings known?

Granted, Lizzy's mistrust of Darcy over the Wickham story is a major impediment, but we cannot know how large an impediment that alone would have been apart from the rest of the proposal train wreck.