When using expdp to create a snapshot of an oracle schema, how do I ensure that no post-export additions are retained upon using impdp to revert?
As always with Oracle, there are many ways to reach your goal. You might want to check out flashback database. This enables you to restore your database in a previously saved state and might be the easiest in your case. Using Flashback Database and Restore Points

Drop user cascade could also do the trick, followed by impdb as you described. What was the problem with the drop user action?

Try flashback database.