I can't really comment on your other questions, but I use flightstats quite often. Although other, more experienced, FTers might have other experiences, in my own experience, flightstats has better/more reliable info than airline websites. It's my understanding that flightstats gets their info directly from flight plans filed with the FAA.

For instance, two days ago I was flying PSP-DFW. Because we were having very unusually horrible weather in PSP (torrential rain/near-freezing temps -- there was actual light snow/sleet mixed with the rain!), I started checking at 9:00 a.m. for a 2:45 p.m. departure. Flightstats already showed a 15-minute delay in departure; AA.com showed no delay. When I reached the airport at 12:30, AA still had the flight listed as on time. Later, there were announcements of a slight departure delay due to inbound delay. The announced info matched exactly what I'd already seen on flightstats. We ultimately left 30 minutes late (plane had picked up ice on the wings during descent; PSP has no de-icing equipment, we had to simply sit and wait for the ice to melt ).

So, no, flightstats isn't guessing. If they say there's a delay, there's a delay.