I would definitely not wait until 30 minutes after e-night starts. The rides are all at the back of the park -- you would be wasting a lot of your e-night. We always go an hour or more before it starts. We either see the parade (if there is one), do attractions without big lines (Jungle Cruise, for example) or get dinner. By the start of e-night, we're ready to hit one of the mountains. Usually, there's a line for the first or second attraction, then almost none.
A tip: often there is a line at the e-night ticket/bracelet exchange on Main Street, but there rarely is one at Splash Mountain photos.
If an e-night attraction is down, a substitution will be made. I think it's usually Pirates.