… You completely missed my post.
I know the duration stacks. That’s literally the only part of the potions that do stack. The CBF is a one time application. 50% more cold resist alone won’t keep a merc alive in my experience. Sure, it’ll help, but it won’t keep him alive by itself.
Thanks for completely ignoring the part about feeding merc health potions from time to time.
At 85 resistance it basically negates all cold damage from the Cold Aura including the melee component.
In normal the cold aura does:
Considering his jab attack does 19-25 damage and smite does 19-22 damage, the cold damage added by the Cold Aura is effectively a 50% increase in damage.
Having maxed out resistance on merc effectively reduces Duriels melee damage by 1/3. Having a defiance merc also helps with avoiding some physical attacks.
100 charges to RW Black for balance 
In general, in the future, you can add a new fault that will give bonuses (damage modifier, ar, ias) to the summoner, depending on the summoned creature. Synergies to make them relevant only for the summoner. Actual assemblies with bows, bashers, etc. may appear.
The branch of summons as a whole is more like a combat masteries, the rest of the builds use it as a support.