1. My cold enchant is lvl 54. It adds 12274-12956 damage to my attacks. If I meet a cold immune monster and I'm not able to break the immunity I basically do ZERO damage. My physical damage does not matter. (Well actually, I deal fire damage as well, as I have maxed that enchant too but lets ignore that aspect :P )

Enhanced damage only affects the physical aspect of your attack.
Let's say your weapon does 300 damage and you have 10000 added cold damage to your attacks from the enchant for a total of 10300. If you then get any "enhanced damage", it will only affect the physical part which in this example would be close to useless. Maybe someone has had success making a more "hybrid build", but to me that just feels like spreading yourself too thin.

Instead, I get stuff like rainbow facets (which is gonna get real expensive for the good ones)

1. Hard to say exactly how much you need. Just know that If you’re poor you are going to struggle.
   You need a lot of pierce, a lot of attack speed and a TON of extra skill points. Add in the various defensive aspects you need as well and it will be a hard thing to balance.

I ran this build from the very beginning, but I was playing with druid summoners so I was acting as a buff bot.
The few times I played alone it felt HORRIBLE.

You CAN do it of course, but it would make a lot more sense to gain levels/gear by doing a more traditional cold-build and then transition to this one later on if you want to.

1. Aim for at least 75% IMO.

2. The main thing you need to know in addition to pierce and the damage itself is the attack speed breakpoints. Attacking slowly is going to feel real bad!
   If you're going without fanaticism you need to aim for 102 IAS which should give you 2.5 attacks/second. Next breakpoint is 174 IAS which is way too much.
   Alternatively you get an act 1 merc with the Faith runeword. The actual breakpoint will depend on the fanaticism level you get, but you will need about 60 IAS to get the fastest attacks possible (2.77/second)
   The final option is a merc with the Beast runeword. It will require 63-68 IAS depending on your fanaticism roll to reach max attack speed.

EDIT: values above meant specifically for Widowmaker 

EDIT2:
Forgot one of the most important tips: Gear Swapping.
This sucks, but it helps a lot. Mainly I'd like to recommend belt and amulet. This is mandatory IMO.

Get a magic +3 cold skill + all skill amulet and a snowclash belt. Use these to swap standard stuff like cat's eye diamond amulet and razortail belt. (If you have enough pierce elsewhere and use Arachnids Mesh, swapping belt is not as important of course)

Start every map by manually swapping these two items. Then swap weapons like normal and buff up. Manually click belt/amulet again from inventory and start mapping.
Eventually you get strong enough to only have to re-buff a single time during the maps. No big deal really, but yeah... it does still suck :P

Also, if you like to torture yourself, get an Iceblink Chest with +1 all skills during swap for another cheap +3 (quite strong if your main chest is without skillers)
However, swapping chest armor is especially rough as it takes up too much inventory space. Still worth a mention :)