Does anyone have any comment on how easy recovery from a flat spin would or would not be for a Cessna 421
No body knows, that's why aircraft meant to be spinnable are put through flight test, with due precaution such as a spin chute to aid recovery should problems arise, and pilot with a parachute should that fail. And it does, test pilot of the Australian GA8 had to bail out when it failed to recover. On exit he fell through the prop arc, fortunately the prop had stopped, and the angels smiled.
The CAC Winjeel had spin recovery issues and the fin had to be moved forward to alieviate. Compare photos of prototype and production.
Some of what may be involved in spin testing.