Not sure this will be an answer per se, more of a long comment.
Re (1) Zourabichvilli (whose book is excellent) ties the concept of event back to Hegel and Heidegger (whom Deleuze mentions in conjunction with DR). Some of what Deleuze is picking up on can be found in Heidegger's "The Question Concerning Technology," i.e., the event as epoch-defining. A key version of what the event is Nietzsche's mad man who declares the death of God (in which connection aside from the obvious Deleuzian texts, one might also look at Heideggers "Nietzsche's Word: God is Dead" as well as §125 of Nietzsche's The Gay Science). This filters through to Deleuze's "third synthesis" in the "Repetition for Itself" chapter of Difference and Repetition, i.e., the event is not something that happens, but has already happened.
Re (2) "possibility" derives from Aristotle's works on logic (collectively known as the Organon), in particular, his Prior Analytics and On Interpretation. Since you're referencing the SEP article, it may be helpful to look at Smith's "The Concept of the New," which was published in Deleuze Studies and which looks at, inter alia, what possibility is and how Deleuze changes it.
Note that virtuality (since that doesn't seem to have been addressed by this question or the other linked one) is a term Deleuze's picks up from Bergson, in particular from Bergson's Matter and Memory. (Bergson is quite readable on his own, but you could also look at Deleuze's Bergsonism)
