Grizzleheim is a side world that has many areas of differing difficulties. The beginning of Grizzleheim is about as tough as Wizard City. As you advance through the world, the areas become more challenging - there's an area that's about as difficult as Krokotopia, Marleybone and Mooshu.
And no, Grizzleheim story does not contribute to the Malistaire storyline, therefore it will not help you get closer to Malistiare.
I am Baldur GoldPaws, a trader from far-off Grizzleheim. I have sailed across the great sea of skies to guide you in all things related to my home world.
As far as level of difficulty is concerned, it seems to me to be on a par with Krokotopia. I like GH simply because of the woodsy setting. The characters also drop halfway decent items. I thought it was fun.