Norway might not add all that much, but given the context of the time, it might not take all that much. Austria and Prussia are hugely superior to Denmark in manpower and industry, but that advantage only counts if you have a long war. A long conflict might well see interventrion by other powers, and Denmark, though driven out of Jylland, would still be able to defend the islands and Norway due to its naval dominance. The loss might simply not look as catastrophic when your territory ends at the Polar Circle rather than Skagen.
Interesting. I always wondered why, post-1900, they were not returned to Denmark after either WWI or WWII. When did effective Germanization become complete, if ever there?
Schleswig-Holstien was majority German-speaking from at least the 1100s onwards. It had come to Denmark through title inheritance and was formally separate from the kingdom itself. For the longest time, that had not been a problem and the court in Copenhagen spoke Low German as easily as Danish. That changed in the 19th century. In fact, part of the reason for the uprising in 1848 and the subsequent war was a Danish effort to standardise its administration and impose the Danish language on a Germanophone population (though it probably would have happened anyway).
After WWI, North Schleswig, the only area with a clear Danish-speaking majority, was returned to Denmark. The significant Danish minority in South Schleswig was given extensive rights (though it was not until 1945 that they could feel reasonably secure in them). Holstein never had more than a small minority of Danish speakers.