A Covenant in Mythic Iceland: What do i need?

With the Regio Hook (page 09/10 , Covenants) , i was considering a gateway to Niflheim.
This would be Niflheim as the Primordial Realm of Cold.
Give it a level 09 Aligned aura (page 16 , RoP: Magic) with Ice/Aquam (rather than Ignem/Cold).
Because this is a gateway regio to a primordial realm , i was considering a Realm Interaction effect
that would reduce Casting Score by 27 (Aura x 03) for all Ignem spells that produce heat and warmth and ReAq to create steam.
Gymir Frost-Father (page 78 , RM) being relevant , though his realm is Jotunheim.
Add have some Muspelli to cause trouble.

Would a gateway to Jotunheim be more suitable?

What other creatures or environmental hazards can i include to discourage the magi?
They have various warming spells and can ward against cold.
None are Ignem specialists , being Mentem (changing emotions) & Healing focused.

I would like to have the Covenant aura enhance an emotion (Favourable Aura , page 16).
This will be a +02 Personality Trait Adjustment , (+03 in the regio).
Due to the association of Phlegmatic with Cold , (A&A , page 40):

Loyal would be the most obvious choice in relation to Covenfolk.
Anyone have any other suggestions?

Loyalty is good if you are looking for only positive qualities, but several of the others are just as valid if, as you imply, the grogs came from the continent to this land of legend - fearful (of the unknown), laziness (from the cold, at least during those seasons), paranoid (or so, from superstition) - or different aspects appearing in different of the covenfolk, as approp to them and the saga. While it's nice to have all covenfolk loyal, the interaction can be made more complex (and upredictable) if some of the grogs/etc. start showing up with other, less desirable, personality quirks.

Whaling may, indeed, fit better than "fishing" per se, but in C&G "fishing" is a catch-all for gaining foodstuffs from bodies of water - salmon from rapid runs, shellfish by diving or simply waiting for low tide, eels (using traps), nets, lines - whatever works, and each area would have their own solution to the local challenge of "fish". For instance, a significant portion (majority?) of medieval "whaling" consisted of simply scavenging stranded whales, and never actually putting to sea to actively hunt the monsters.

Altho' medieval sources discussing whaling are scarce (and so much is guesswork), whaling is a nice touch because not only do whales produce food (if a rather oily and pungent meat) and bone and teeth for art, but believably oil* (for lamplight, important in those long winter nights) and, if sperm whales (perfecty legit in this area), ambergris, which was recognized in medieval times (and before) for all kinds of beneficial properties. (It was often found floating or washed up on shore as well as extracted directly from caught whales, possibly more often, so that's a plot element just waiting to be found.)

(* Possibly - I find sources unclear whether or not whale oil was definitively used this early, but I find it hard to believe that anything so useful would be overlooked. Dunno, but it doesn't feel wrong.)

And, of course, the adventure hooks they bring: The Unnatural History of Monstrous Medieval Whales - a bit academic and dense to read, but not overly long and good inspiration if you can wade through it.

An interesting choice for pertsonality could be "prudent". This would avoid grogs getting into trick troo easily, and ebign bashed for reckless behaviour, but would also mean that much forethought might be put into actions by them. Too much, even. And that they will look for the least risky approach to things, soemething that might be at odds with the demands of their masters.

Cheers,
Xavi

Not a bad idea at all.
Have one "clever" grog who also helps the others with all their meticulous and "prudent" planning for any occasion.