elves and also about the sea? What connects mythical and magical Alfheim with the factual and formidable North Sea? People. The same people who live by and travel across the sea also populate their environment with imaginary beings - and sometimes see them. My Northern Irish aunt-in-law saw horses and riders in the waves of the North Sea. I have no reason either to doubt that she had that visual experience or to believe that a film of the waves would have shown them taking the forms of horses and riders. Brains process sensory impressions in diverse ways.
Why does offering to Freyr lift the elven spell from Hadding? The elves were afflicting Hadding with bad weather and:
"...Frey...became lord of the weather..."
-Roger Lancelyn Green, The Saga Of Asgard (Penguin/Puffin Books, Harmondsworth, Middlesex), p. 40.
Presumably a god can overrule elves? Later, warlocks and the Wanderer waged war with winds and weather. (pp. 196-197) Later still, Rupert Bear helped Elves of Autumn to apprehend one Imp of Spring who refused to hibernate and instead created pockets of spring in the Nutwood Autumn.
Freyr is a Van but where did the Vanir originate? The Aesir, like the jotuns, originated in Ginnungagap but the Vanir are a different pantheon and would originally have had their own distinctive mythological cosmology. I suspect that Green invents some details to tie his narrative together but, wherever he got it from, his account of the Vanir is as follows:
"The Vanir were the shining ones who had been born out of the upper air and dwelt first in Uhland, above the high top of Yggdrasil. They never came to Asgard nor Midgard, nor ever even set foot upon firm ground..." (p. 35)
If they "...dwelt first in Uhland...," then maybe they later moved to Vanaheim which Anderson describes as:
"West of Asgard..." (War Of The Gods, p. 9)
In one mythological passage of "Star of the Sea":
"Out of the east, the morning behind them, rode the Anses into the world." (Time Patrol, p. 557)
The Vedic sky gods who became Anses/Aesir came west with the Aryans.