I have a theory on how it can work here. It's all about the climate where the creature lives. As ironic as it may sound, if this thing were meant to live in volcanic areas, the idea of resisting fire and rocks would work (not even if the pokemon lived near fire, fire still melts ice and burns bugs, rocks are stronger than ice and shatter it and squash bugs). It's high above GROUND level (but theres still ground there and it will not always battle in its own enviroment), and away from the plants that are unhealthy for it (WTH), but not high enough to need to handle the cold of high altitude (its an ice type it resists ice, not weak to it). Steel fits in there... somehow. It's steel, so it doesn't matter too much.
all your arguements are quite bad
Continuing to explain why Inverse would be better than Ice Shell, which could probably be given a less exclusive sounding name (whats wrong with it being exclusive?), is the fact that after you've switched in, you can give it away. I'm talking Skill Swap, and it can see big results!
only psychic pokemon get Skill Swap, dont go there.
You can skill swap many things and they wont like being Inversed. Gyarados, for instance, wont like switching in to gain a weakness to STAB bug, not to mention how Skill Swapping will give him an intimidate (though you'll have one as well.) Heatran is a big one here.
Skill Swapping Heatran gives you fire immunity, and lets you wall it unless it carries a rock move or Luster Cannon. Let's not forget that both STABs would be 4x effective.
Magnezone won't like being Magnet Pulled while being weak to both STABs either.
Come to think of it, Inverse can be a very annoying ability to skill swap. A few creatures dont care, or they really like the swap.
Mamoswine likes Skill Swapped Inverse. Having 5 resistances, electric immunity and only a weakness to poison? Great! It also has STAB Ice Shard to smack you around before you swap it to him. With such poor defenses, that'll hurt.
Abomasnow might like the swap too. It trades its weaknesses for its resistances... though its resistances are to common types. It can't really hit too hard back as grass, ice and ground are all resisted by this creature naturally. Too bad for the snowmaker that the swap happens after SR damage.
how would a bug/ice get skill swap?
Hmmm.... Inverse + Skill Swap might be too powerful. I guess prediction can beat it, though the idea of switching a dragon in on an ice type is very strange. Salamence will get himself intimidated, and Sandstream may turn against Garchomp. These are of course rare instances of how the switch may help the frosty insect, made even more rare by the fact that, yet again, no dragon switches in on an ice type.
Hate to sound like I'm derailing a topic, but I just feel like adding my two cents. Feel free to make this thing sound less broken because I'm not much a battler, just a concoction stirrer.
cooper has asked us not to sugest new abilitys and your idea has worse logic than the mountinear ability.