I figured out the User timing question - I had inserted a W3C User Timing mark onto the page I was testing and realized it outputs that user timing. - see http://www.webperformance.io/custom-user-timings.
The browser’s main thread is considered “interactive” when it is not blocked for more than 50ms by any single task so it will be able to respond to user input within 50ms. An interactive window is a period of at least 5 seconds where there are no main-thread tasks that take more than 50ms.