It wasn't coffin corner. This was a Lear at reasonable altitudes, not a U2 at the edge of space. The death spiral suggest a loss of control authority. It is conceivable that one wing severely iced to the point of unrecoverable asymmetric lift but I think it was something more the icing. It might have lost part of its wing, or the controls on one wing locked into an unrecoverable position.

In the pictures, the flaps appear present but not extended. If they had been extended they would have ripped away at that speed. And they would not have been able to lower them after the overspeed began. I think the answer is in the control system. One or more control surfaces were doing something other than commanded.

Some images seem to indicate a loss/absence of the horizontal stabilizer, but it is hard to tell AI from real these days.