What’s the best weapon for an act2 merc for a blizzard sorc? Insight? Reapers toll?

Probably the typical guillames treachery eth reapers toll with might merc.

Sometimes I'll go Infinity to save a ton of skillpoints on cold mastery, then use those skillpoints for something else like fireball.

How many skill points from cold mastery can I save by using an infinity?

I would just put 1 point max in cold mastery if I am going with infinity.

What rognar said.

Gives you lots of options, you could do ES, fireball, nova, enchant. A light sorc makes better use of it but I just like how blizz handles versus lightning spam.