Is a light sorc viable p8 from start to finish or only really after obtaining infinity? Better to go fire, cold, then light?

only after infy
before infy you want orb+fb

I did it in a wt once. Doable but not recommended.

Yes viable, though its good to get secondary elemental skill such as blizz, orb, fb, meteor to kill trash monsters with high light res. You can also shop an LR wand from people that sell necro wands.

I always preferred blizz if no infinity

semi-viable. you'll need a beefy merc since there's so many light immunes otherwise there's little hope if you don't skip mobs