Just my two cents, but I believe that if the Confederates at Ft. Donelson were allowed to fight and they fought as well as the small battery company at Ft. Henry, things would have gone well....at first. I do not believe Grant could have forced an early surrender. The Confederate troops were surprised and demoralized by the sudden defeat.

The short term odds were in favor of the defenders IMHO. And I also believe there would have been political or military pressure for reinforcements. The Condederates invested too many resources in those forts just to cave in so quickly.