Can I save an undercooked loaf of pumpernickel?
When a loaf is baked the interior steams. The water in the dough reaches boiling point and gelatinizes the starches in the flour. After baking the dough has largely dried out. While this might be less true for very moist breads such as pumpernickel, I doubt that baking again will have much effect on the remaining uncooked flour. So I'm afraid there is not much you can do at this point.

Toasting may make the bread more palatable, but it won't make the essential chemical change that you need.