Q1: A bizzard sorc can do her job with MF stuff mostly everywhere. MF beyond 300% doesn't make much sense with respect to uniques, it's still useful for set and rare items. She's most effective in the Tunnels and all you need there is a bit lightning and (because of death explosions of cold-enchanted bosses) cold res, perhaps also poison res for occasional hits by a zombie. I'm using Tal's armor+amulet+belt, shako+ptopaz, Ali Baba+MF/res jewels, Rhyme, Goldwraps, Chanceguards, rare boots with FRW, MF and resists, Nagelring, rare ring with fcr and resists, as well as skillers and resist SCs. On the weapon switch there's an Oculus and Spirit. I could use a 4 Ist monarch or a 6 Ist sword or 2 Ist Ala Baba instead, but that would increase MF from perhaps 450 to 650 which doesn't make sense. These two would be better for an act boss killer who doesn't have much MF otherwise.
Q2: I think a holy freeze merc + Insight is more effective. MF is usually done in a L85 area at /p1 (Tunnels are best for a blizzard sorc IMO) because bosses don't drop more on higher /p settings and you don't need more power on the offense there. Those few cold immune bosses are easily killed by the merc.
Q3: Spirit is for the non-MF weapon+shield slot, together with an Oculus or perhaps HotO. You don't need it for MF. I prefer Rhyme because it has CBF. If your merc has to hold the line, you want to move by running rather than by teleporting and cold damage would slow you down without CBF. If there's nothing to do for her but MF, you might even use a non-MF shield with a lower str req instead of Spirit and have quite a few more points in vitality. I prefer Spirit, however.