How to automatically redirect to first item of second navigation level in a multi-level menu?

You can use the ‘About Us’ page both in the 1st level menu and in the 2nd level menu. So essentially the same page is in each menu, just rename the one as 1st level menu to ‘About’.

The ‘About’ page should still have a redirect for the Search Engines.

Keep in mind that this way things like the class .current-page-ancestor are not correctly set on the ‘About’ menu item when on a sibling page like ‘People’