To answer your questions:
1.) Those miles were accumulated in the last 13 months. That is the period counting for a status upgrade. You are indeed 8,631 miles away from gold, but remember that the oldest status miles will expire after 13 months, so depending on your past and future flights you could need more than 8,631 miles.
2.) The 27,095 miles were collected between 01/05/17 and today.
3.) To be precise, they expire every 13 months.