Why style the '2' with Roman numerals instead of a number? Because Frozen II can also mean Frozen The Second, which ties as a hint into the sisters' royal blood, since heirs to kings and queens had the Roman numeral after their name if they shared a name with an earlier monarch.
When Elsa's running on water to fight the nokk, that platform is heaving up at an ever-increasing angle and is made out of ice. And shes soaking wet. Those things combined create a nearly impossible climbing situation without spiked climbing boots. But Elsa has ice powers, so her magic can keep her from slipping on ice. And it also explains how she managed to climb the North Mountain without any gear.
The reveal that Elsa has given Olaf a layer of permafrost since the end of Frozen explains why exactly he was able to roam around the castle in Olaf's Frozen Adventure without his personal flurry.
It's revealed that Elsa wears her mothers shawl whenever shes worried about something and that Anna recognizes this as a sign of Elsa being anxious. So this must have happened before for Anna to piece it together. One day she walks in on Elsa stressing out about something and shes got something unusual wrapped around her shoulders. Its a shawl, and Elsa doesnt need to wear shawls. She doesnt get cold. And this shawl looks familiar. And then Anna remembers her mother wearing something similar...