I think there are two main types of deviation points from plot: one, the points where something important happens between Darcy and Elizabeth (the most important example, as you said, is Hunsford, but also the Meryton assembly, or the first meeting with Wickham, or the Netherfield discussions, or the Lambton/Pemberley meeting). The other type is one of the moments where some coincidence happens to bring them both together again - removing one of them will change canon and allow for new plot with an original way of finding HEA. The letter not being read, the Pemberley meeting missed, Lydia not being rescued, Lady Catherine not interfering are typical examples. Of course the latter stories usually feature prolonged angst and more complicated resolution.