Which Lapras to power up

Probably after the gym rework.

FB/IcB or FB/BLZ. Water moves are irrelevant against most Pokemon which are weak to ice, and ice is weak to most types which are weak to water.

I would wait till after gym rework...

I read somewhere (SR probably) that they mined phrases like your pokemon successfully defended the gym or what not...which may translate to "your" particular defender actually knocked out an attacker...which may then translate to getting bonuses or other things needed to get coins/dust, whatever...

If that's the case, things like quick hitting charge moves like Stone Edge, Hydropump, etc...may be worth "more" to powerup as defenders...

If you're at a gym and the attacker always beats you without missing a beat, maybe you get "0" pokecoins that day...

AGAIN, all this is a wild guess so best bet is...

...Wait for the gym rework...everything will be answered when everyone can test/play it after....