1263 - Haakon VI leaves Bergen with his war fleet in late spring, as soon as the weather improves, doesn't tolerate Alexander's delay tactics, and attacks midsummer. The Battle of Largs (or wherever) is a decisive victory for Norway, a right proper trouncing. However, Haakon realizes the futility of trying to keep mainland territory if the native highland clan isn't aligned to the Norwegian Cause. The new Treaty of Perth concluded before October or the following April, after Haakon survives the winter in Orkney, sells/rents the Argyll peninsula (turned Scoto-Norman in 1250) and the Isles of Clyde to Scotland; but keeps Kintyre, Knapdale, and Lorne still happy with Norwegian suzerainty. Likewise, Norway keeps the Hebrides and Mann with a proper appointed Jarl of Sudreyjar to reduce the clan feuds, which Magnus VI's new law code (crimes are offenses against the state) will soon curtail. Most importantly, Haakon lives at least until 1270, dying around 65 like Henry III, to conclude trade agreements and defensive treaties... and build more ships
1270 - Magnus VI doesn't stop having kids after an heir and a spare, doesn't die in 1280, and lives until at least 1305, dying around 68 like Edward I, after Erik and Haakon are both legal age (21) and Norway has formidable fleets (plural) in the Baltic, North, and Irish Sea. Immigration incentives -- Don't want to Normanize? Nordicize!
1282 - Llewelyn ap Gruffud doesn't get tricked into dying at the Battle of Orewin Bridge, trounces the English, and at least keeps them busy in Wales for a few more years. Edward outright breaks Princess Eleanor's betrothal to Alfonso of Aragon, or at least swaps her for Joan of Acre, when Peter and Constance are put under papal interdict over Sicily and negotiates her marriage to Prince Alexander instead. The Papacy loves England, so the betrothal annulment/swap and dispensation for consanguinity are promptly issued. Haakon, Duke of Norway is betrothed to Richeza of Denmark, firstborn daughter of Eric V Klipping. The Papacy loves Norway, so the dispensation for consanguinity is issued. Agnes Haakonsdotter is legitimate, Um, Harald, Duke of East Norway is betrothed to first born Ingeborg Magnussdotter of Sweden (she married Eric VI Menved OTL). The Norwegian-Swedish alliance against Denmark and its constant Slesvig-Holstein-internecine issues strengthens and Danish influence in the Baltic diminishes.
1283 - Margaret Dunkeld, Queen of Norway doesn't die in childbirth. She and Erik have plenty of sons and daughters -- none of whom become heirs to the Scottish throne but do keep the Norwegian monarchy and Jarldoms of Nordreyjar, Sudreyjar, Iceland and Greenland--subsequently navy and trade--going strong.
1284 - Alphonso, Earl of Chester lives to marry Margaret of Holland and have plenty of sons and daughters with her. Edward of Caenarfon is stillborn, and Eleanor of Castile dies in childbirth. Edward marries Margaret of Flanders. Thus he and Guy don't plot Floris's assassination and proto-Belgium and Netherlands continues to pull away from France without getting completely sucked into the HRE, and the Hansa remain more heavily west of Jutland. Alexander IV doesn't die and has plenty of sons and daughters with Eleanor of England--no succession crisis or war of independence.
1286 - When Eric V is assassinated (4th in a row) in 1286, the Danehof elects Haakon king of hereditary Denmark based on the claims of his mother and wife, halting the "Age of Decay". The Danish royal House of Sverre-Estridssen flourishes. Magnus VII Erikson, junior king of Norway, marries Margarethe Eriksdotter (aka Martha of Sweden in OTL). The Norwegian royal House of Sverre continues to flourishe.
1299 - Erik II doesn't die until Magnus VII is legal age with an heir and a spare. Harald and family establish the hereditary Kingdom of Iceland, which administers the Greenland colonies and remains closely tied to Norway, Sweden, and the Isles, which now includes Sudreyjar, Nordreyjar, and Faereyjar... possibly Gaelic Ireland. Forming a Scottish-Welsh alliance, Prince Alexander marries Gwenlian ferch Llewelyn of Gwenydd (captured as an infant and imprisoned her whole life in a Lincolnshire convent OTL).
1306, Birger--oops!-- dies during the Hatuna Games because Norway and Denmark don't intervene. Thus, his more competent brothers aren't captured at the Nyköping Banquet and starved to death.
Erik is elected king of hereditary Sweden and Valdemar still becomes Duke of Finland. Erik "XII" the Wise marries Ingeborg Eriksdotter of Norway and Valdemar marries Ingeborg Haakonsdotter of Denmark. Both Swedish Sverre-Bjelbo lines flourish and the Kingdom of Finland is formed, begins re-absorbing Karelia from Novgorod, but Scandinavia learns vital republican lessons from the proto-Russians. The Nordic Commonwealth forms with internal autonomy, similar tailored domestic policies, common foreign policies (blend of bloc merchantilism and capitalist free trade), and top-tier Kings Council (rather than one king, which killed Kalmar OTL). Most importantly, Scandinavian Royal and Merchant Navy is HUGE, totally dominating--but sharing/taxing--the Baltic, North, Irish, and White Seas and North Atlantic/South Arctic, especially after absorbing the Pomors.
Great Famine and Black Death less devastating for the Nordic Commonwealth due to larger interrelated aristocracy that isn't fighting each other all the time, immigration policy, and mobility... and eating fish rather than bread!
Heavy recruitment, religious/ethnic tolerance, and bilateral expansion while remaining as neutral as possible in Anglo/Continental European affairs sets them up for an Arctic Empire during the Age of Discovery, especially as they'll likely have already colonized "Eriksson's Bay" and possibly reached Alaska already because their indigenous and transplanted population is already uniquely adapted to the frigid polar Pre-Industrial climate.