What is the best way to create a slideshow in iphoto across multiple events?
Even easier: Make an Album of the photos you want to use in the Slideshow, and base it on that. This has a couple of advantages:


It simply references the photos in the Events, and uses no ezra disk space, while keeping your Events intact
You can sort the photos manually, into any order you like, which you can't do with Events.


To make a new Album: select a couple of the pics and go Command-n; then drag the rest of the photos to that

Or, shift-command-n to make a new empty album