Conviction and Lower resist work at 1/5 of their effectivenes against immune monsters.
So, a maxed conviction (-150%, lvl 25) would subtract only 30% resist from immunes, and a very good lower resist (-65%, lvl 26) would subtract only 13% resist from immunes.
So, you can subtract a total of about 43% from immunes.
In hell, tons of immune to cold enemies have very high resist to cold. For examples, in act IV and V:
IV
Grotesque: 160% cold res
Grotesque wyrm: 160% cold res
Stygian hag: 150% cold res
Stygian dog: 150% cold res
Oblivion knight: 180% cold res
V
Blood boss: 150% cold res
Blood bringer: 120% cold res
Blood lord: 125% cold res
Bone mage: 160% cold res
Cave leaper: 100% cold res
Dark archer: 140% cold res
Dark lord: 135% cold res
Death brigadier: 145% cold res
Death clan: 165% cold res
Death slasher: 135% cold res
Enslaved: 130% cold res
Flesh spawner: 145% cold res
Foul crow: 120% cold res
Frozen abyss: 210% cold res
Frozen creeper: 150% cold res
Frozen scourge: 170% cold res
Frozen terror: 160% cold res
Ghoul lord: 130% cold res
Grotesque: 140% cold res
Hell clan: 155% cold res
Hell lord: 135% cold res
Hell whip: 160% cold res
Hell witch: 145~160% cold res
Ice boar: 140% cold res
Ice spawn: 150% cold res
Infidel: 150% cold res
Invader: 160% cold res
Lasher: 130% cold res
Night clan: 135% cold res
Oblivion knight: 140~145% cold res
Overlord: 145% cold res
Overseer: 120% cold res
Pit viper: 145% cold res
Plowling dead: 175% cold res
Reanimated horde: 150% cold res
Rot walker: 130% cold res
Siren: 125% cold res
Slayer: 130% cold res
Slinger: 150% cold res
Snow drifter: 150% cold res
Spike fiend: 100% cold res
Tomb creeper: 120% cold res
Vile witch: 155% cold res
And so on...
As you can see, only a minor part of cold immune monsters, in the critical acts of hell, can be deprived of cold immunity. Even with the conviction + lower resist combo...