Snowbird unless Thor uses Godblast. As even without transforming Snowbird showed she can trade blows with Hercules and swat away his punches with little diffuculty (Thor and Hercules are said to be physically the same). Then if she transforms into Neooqtoq the Great Beast she gobbles him up as in that form she was taking on thousands of gods (and gods that were hurting Atum) and was winning.
Same, it's just her "true" form as she really isn't a blonde white female.
Nope he most definetly wouldn't, but frost giants are strong but not skyfather strong. Surtur is, but he also at times has wrecked Thor). What's interesting is Surtur may have some kind of connection to the Great Beasts as Tanaraq now has control over Surtur's armies.