Yes. The number is subtracted from the monsters original res (if it is not immune) and the minimum resistance can be reduced to is -100. Many monsters have cold resistance higher than 0 and lower than 100.
There is a point, but it gets less and less so the further you go past 100. Almost all regular monsters won't have cold resists over 50 unless they're immune, so past -150 you're really not gaining much more.
The one exception is if you plan to run Mephisto a lot, he has IIRC 75% cold resist, so stacking up to -175 is worthwhile for him.
I'd just put 1 pt in CM and let the +skills do the rest. It doesnt matter if they're hard or soft points, and -ECR from CM works exactly the same as -ECR from gear like Doom, they just add straight together.
Doom isnt worth it because you already have -ECR from Cold Mastery, and the most a monster can get is -100. If you have, say, +11 skills (a modest total for an endgame sorc), then you're already at -75 ECR from just 1 pt in CM. If you have a Doom with -50% ECR, then just half of that (25%) is actually doing any good against monsters with 0 cold res to start, and the rest goes completely to waste. And that's just with 1 pt in CM, you probably want to invest more.
What cold sorcs need is +% cold damage from places like Nightwings and Fathom and +5 cold facets