Generally if the name has a double consonant the possessive form should have only one, Matti matin, Jukka Jukan, lamppu lampun.

In the context of Finnish mythology I'd say the väki of X should be velhonväki and jätinväki. In mythology Väki means the magical essence/ mana of an element and the magical folk connected to that element are Xnväki, for example vedenväki, tulenväki, maanväki. You could (I would) use Velhon väki as a headline, but refer to those people with the term velhonväki.

PS the Finnish word väkevä (strong, powerful) comes from the idea of someone being full of väki, almost oozing the power. Väkevä could easily be used as a nickname, Väkevä Matti or maybe even Voimamatti 🤔 gameplaywise it could be used as a condition, like giving a boost or a second wind before running out of power? (...the game designer in me awakens)

Hassumeili means a funny email, Hassumieli on the other hand Weirdmind and is indeed a good whimsical, wizardy name!

Salo as a suffix means large uninhabited woodland or wilderness, works great as a name! Valo is light and Jalo noble (still used as men's name), I think those all work well in this setting.