Well, a couple things. Brittling doesn't have flat-footed as an effect. It makes the target lose their dex bonus to AC, rather than making them flat-footed. I don't mind allowing a reflex save so much, but keep in mind that it's a standard action and only ever does 1d6 damage. You're trading your action, capable of doing a lot more harm, in exchange for removing someone's dex bonus to AC and possibly halving an object's hardness for 1-4 rounds. Magic Missile does more damage, and doesn't require a reflex save. And as I mentioned before, this is not even instilling the flat-footed condition.
All the other changes I can deal with, except the complete removal of Frost's Bite, rather than spreading out its effects over the course of the class. Cold resistance is pretty common, and as energy immunities go, as I understand it Cold comes right under Fire. Ignoring hardness is meant to reflect a coldness that is so bitter that is makes objects more brittle, and easier to break (as happens IRL with such levels of chill on many materials). I would suggest changing it rather than removing it, so that at level 3, it overcomes an amount of cold resistance equal to the caster's HD,
with Cold immunity being treated as Cold Resistance 25 (so that at level 3 when the ability is normally gained, you're still unlikely to do any damage to a cold-immune creature, but at level 13, you're actually able to make a dent because your cold powers are so much more frigid).
(changed idea, see below). Then at level 8, cold powers interact with hardness on the same level as acid/sonic (ie: dealing normal damage, reduced by hardness as normal, rather than 1/4 damage).
I think that, as a scaling bypass ability, it's more than appropriate and fair. And if you still think it's too much, I could do without the Skate line of abilities, since I'll have flight anyway. If the class were being taken as a base class at level 1, then I'd agree that it's too much for that level, but this is being taken starting at level 7. My character won't even have the ability until next level (ECL 9), assuming we make it that long.
And one question about your change to never-melt ice: Would you say that an ice creation wouldn't melt unless hit by fire damage, or is it that it still melts regardless and just
doesn't take 50% more damage or so from fire?
As for all the changes, especially to say Wintersmith, please keep in mind what level I'm actually getting those benefits. Wintersmith doesn't come until level 19
. I don't expect to have the level 15 and 17 upgrades since it comes far after I'd achieve those benefits, so I don't mind those being stricken. But as for the change adding the resist/immune bypass at that level, please see my above idea.
Not changing my opinion about frost's bite, except for having it mirror the Fire Mage's "Piercing Flames"
Piercing Flames (Ex): From 3rd level on, a Fire Mage's Fire cuts through Fire Resistance, hardness, and Immunity. No more than ½ of the damage inflicted by his fire damage can be negated by hardness or immunity or resistance to Fire. In addition, the Fire Mage ignores the first 5 points of Fire Resistance that a target has.
Rather than having it be this exactly, I suggest doing the ignore resistance up to = HD, then follow the quote ability pertaining to immunity (dealing half damage to cold immune creatures). I'd also suggest the other hardness solution I came up with above, where it doesn't bypass hardness, it just does normal damage vs. objects rather than 1/4.