Even if you aren't using any cold spells it's still the best passive to get imo, just give your follower a cold weapon and get a 20% dmg boost
Absolutely not. Followers tend to die, and generally hit one target at a time.
Then you haven't been gearing loving your follower enough. My Enchantress hardly ever dies with about 40k health in act 2 inferno and only occasionally in act 3/4. To keep everybody more chilled a faster attack speed weapon is better, as well as rings/amulets with IAS.