A.) Because Air Elementals are extremely exploitable in a Storm. They also have an armor piercing attack and can fly. Storms are already so exploitable, making them more so by chainsummoning would not 'diversify' the game. Once again catapulting Air magic to the forefront of synergestic spells.
B.) Because by adding a vulnerability it is quite a sacrifice to try to get full resists (thus making it a true SC). Ice Devils are already physically more powerful than Arch Devil's, have better buffs. I don't see why the 'diversity' is hurt by having them equally niche useful as an Arch Devil as far as resistances. In actuality you may still always choose a ID over a AD (if you have the option) because of the physical stats, but depending on who you are fighting it might have a counterweight.
a.) The protection that normal ID's lose even in Heat 3 Enviroment (outside of the ID that is especially sensitive to heat) is minor and acceptable when you can equip a Ring of Fire to null the Encumberance issue (which is the big issue with Cold Bloodedness).
My response might be misunderstanding. I can't fathom how having one unit with a different vulnerability to it's opposite element lessens the diversity in anything but equipping the Ice Devil. I can say in the stock game before the nerf, there was never a reason to pick an AD over an ID with the exception you couldn't gather up enough Air gems to make Boots of Flying. As it stands now if you will probably summon the ID's first but they won't be immediately invulnerable to 90% of the mages you meet with two items, instead it requires 3
