Doubly Ugly, you refer to the recent accident in Buffalo.
I would suggest caution in any ‘momentum’ analysis as the facts leading to the accident are far from clear. The aircraft was flying the approach in icing conditions, but beyond that there is only speculation.
A tail stall is a rare and somewhat unique problem in some aircraft, which should not normally be encountered when flying within accepted safety boundaries. A tail stall would not necessarily (IMHO most unlikely) result in a spin; ‘conventional’ or otherwise. However, this possibility should be excluded, but if encountered would probably result in a high vertical descent rate. A flat spin is a rare form of spin with results similar to your description, but involving all 3 axes. I do not know if the aircraft type is susceptible to a flat spin. The reported facts add little support for any of these outcomes.
From a physics aspect, this and similar accidents involving icing might have more relevance with super cooled liquid water droplets transitioning to ice when striking the aircraft.
Re momentum, see
Chapter 18 Stalls and Spins.