Any thoughts? This is a problem that I can see causing me many more headaches in the future.
Have you resolved this issue?
In order to allow for you to embed credentials you have to have manual entered them. In other words you cannot use windows authentication. For where I work it means we had to set up proxy users (TableauReadOnly users) to our databases so that we could pass through credentials on to tableau so it could refresh the data sources. Tableau cannot use credentials it hasn't been given. Also, it has to be allowed by whoever is your server admin.
I haven't found anywhere that has expressly said this on Tableau's website, but I go over this topic almost daily with coworkers who are using tableau.
here is a decent link on how to embed credentials
There is no way to set just one extract to refresh, as far as I know. However, the refresh "failing" doesn't necessarily mean that your data isn't updated--your extracted Oracle data may still be refreshing, while the Excel sheet is "failing". So, you may be getting the results you expect.
In situations like this, I prefer to pull the Excel data into a SQL view--I'm not sure if you can push the Excel data into your Oracle DB, but this would make life easier for you.