I like your writing, it sounds plausible.
Thanks
.I´d suggest you take the compromise solution and later on change it into the best case scenario, just gradually through 1-2 centuries. If the nobility of Norway Sweden and Denmark interacts and intermarries you won´t have much difference culturally between them
That is the general idea, yes. The tricky part is to flesh it out...
Suggestion:
The idea of scandinavians eventually settling Canada or east coast USA is nice, and often used. (or atleast pondered upon).
IMHO the population base is just to small. There will be changes in North America, but dont expect to see a Scandinavian Empire there...
How about letting them be more interested in European matters. In fact the shortsighted monarchs of Denmark were usually trying to get rid of Iceland offering to sell it to Britain on numerous occasions. (Even the Hansa league at some point). Iceland isn´t though completely insignificant since it supplies a whole lot of fish, which was very valuable in the middle ages during the fast.
How about the king of England actully accepting Iceland in exchange for something. (Maybe as a dowry?) and letting the union focus on the Baltic sea, Russia and Germany.
Sorry but Im planning the quite opposite. IMHO the key to succes is to stay the hell out of Germany and keep Icland as a key initial stepstone for a maritime power. Baltic and parts of Russia, yes. But Germany is a definite no no...