Some mechanical questions about mp abilities

Regarding #1 the main differences:

• Snap freeze can debuff better (it has an extra 20% universal debuff that cryo lacks)
  
• Snap freeze is a ~1 second cast with shorter range but basically instant effect on everything it hits (protip: spray the entire area if there's multiple foes nearby), whereas cryo takes a lot longer to apply it's effects
  
• Cryo has superior range and potentially shorter cooldowns if you spec it that way and/or stop before the entire cast is completed
  
  

You should then like Human Artificer, even though she's a bit on the squishy side, she has Invasion and Cryo Beam + Remnant Cryo Gauntlet. I love her she can make pretty glass explosions all day long and help others by doing those.    

And cryo ammo from your assault turret too. My engineer is all ice.

I just wish he could use a remnant cryo gauntlet

Invasion is great debuff skill, maybe the best. Lounch it at bigest enemy in a spawn group from the other side of the map, let it spread, benifit.
Rinse and repeat.
Leave VI at 1.

If you haven't figured it out already, Cryobeam is kind of like a cold equivalent of flamer. Hold down your power button for longer uses to deal more damage. From what I know, Cryobeam has two uses (which have kind of already been covered). Snapfreeze evolution for Cryobeam works wonders on groups (particularly red bar/health based characters). The burst damage that you can deal with specialization off the charts. The other evolution, Cryo Trap, is great for creating "hazard zones" that can freeze enemies funneled through choke points, even when you're not actively using your Cryobeam. Right now I have my Human Enginner specced for Snapfreeze to take advantage of the fact that I can detonate combos with Overload. And I have my Artificer specced for Cryotrap to be a more general debuff character.

Snapfreeze (the ability) is pretty similar to it's ME3 counter part. While it does sort of do damage over time, once you hit the power button once you commit to the full animation. Unlock Cryobeam it freezes unshielded targets on contact. It's great at disabling red bar enemies and debuffing (it has one of the best debuffs in the game). As an added bonus, you can still trigger cryo explosions should you need the burst damage.