mm43, I really like your analysis. It fits with other known data very nicely. The impact point, for example, is within the annular ring I'd expect for minimum distance from last known position and maximum distance from last known position for a plane that was not spiraling down completely out of control.
No hand of God reached out to stop them cold in their tracks. And they may not have been entirely in control. The distance of your calculated point of impact is near a point I'd expect for a plane that had been going at 4 to 5 nm a minute and was turning to the left and downwards.
Now, as to why it might be turning to the left I can only guess from an ignorant bystanders standpoint. I understand that normal deviations are to the right for traffic control reasons. I have further gained the impression that the planes fly slightly to the right of the nominal track under normal circumstances, although this is not critical to my thinking. The pilot in charge may have deviated to the left planning to go down to an appropriate flight level for a quick return to Brazil for some reason. And then he lost it. Or the entire trip to the ocean may have been in only partial control leading to the deviation to the left.
Maybe he'd executed a partial turn, the engines quit, and he dove to try to restart them. He got too low, tried to pull up into a proper ditching, and didn't made it.
Those are raw almost fantasy reconstructions to fit this additional data consistency. So please don't make a big deal of them.
There are still so many possibilities that it'll be really hard to place solid "blame" on any one thing. But, we may know enough to call for some changed procedures and MAYBE some changed equipment to prevent this happening again. (Personally I'd like to hear that the flight control computer was modified to "very strongly recommend the pilot take over but retain control until the pilot is firmly at the controls" rather than throw up its hands and say "The pilot's got the plane" when the pilot is having trouble due to buffeting getting his arm into position to take control.
JD-EE