What POD would be best in regards to the goal of a unified Scandinavia? And just out of curiosity, who long could a unified Scandinavia exist?
And I'm meaning something besides the Kalmar Union (perhaps a stage up from the Kalmar Union)
There was a fair share of Pan-Scandinavism in the 19th century. Probably a direct consequence of what was going on in Germany and Italy. And then there was the problem of Frederick VII of Denmark having no heir, but already while he was Crown Prince in 1847, he did appoint his great-niece's son, Christian of Schleswig-Holstein-Sonderburg-Beck as his heir, ending that problem. Hope begun as the liberal Oscar I of Sweden ascended to the throne, and offered Denmark support in its war with Prussia in 1848, but ended after Charles XV refused to enter an alliance with Denmark in the Second Danish-Prussian War in 1864. If my memory of 19th century Swedish history isn't failing me, it is Louis Gerhard De Geer who is usually credited with keeping the King from joining in.
Irrelevant:
(De Geer is for the record remembered as being the first Prime Minister of Sweden, though similar offices had existed before, such as the Lord High Chancellor which was succeeded by the President of the Chancery, eventually succeeded by two offices both called Prime Minister (Prime Minister of Justice and Prime Minister of Foreign Affairs). The entire thing is a consequence of that we Swedes, unlike the British are fond of writing down, then tear up, and re-write parts of or our entire Constitution (1634, 1719, 1772, 1809, 1974). Scandinavian constitutional business is tricky stuff. They are still debating whether the Letter of Union establishing the Kalmar Union was a legitimate document or not, which more or less puts the validity of everything thereafter on a coin toss.)
In any case, perhaps you should have De Geer get a little of those Pan-Scandinavist feelings... Have him support Denmark, and have Swedish regiments and the Swedish Navy head South to smash those Germans as they go imperialist-expanding in Danish territory. This will make it much easier for the population, caught up in nationalism and Pan-Scandinavism to later on accept a personal union or even a formal merger of the countries of Sweden, Norway and Denmark into some kind of federal monarchy.