First question, is apocalypse actually viable without sunders and such? I've hit 88 with abyss, but mainly run cows now. How does fire do there for kill speed with same amount of + to skills?

Do + to fire skills work with it since its techniclly a chaos tree, not a fire tree?

No.

Yes.

No.

Flame wave is generally more efficient than apocalypse, you only really need apocalypse to death with some very high hp broken immunity mobs or when you're undergeared and stuff doesn't die in 1 flame wave
A sunderless fire warlock can pretty efficiently kill areas without the few mobs that don't get broken with a conviction demon- chaos and baal are the obvious two. You can't kill venom lords or lister, only merc+demon grind them or lure them
Once you have sunder, a fire warlock is a bit slower than an echoing strikes build without top tier gear, but with top tier gear a fire warlock actually exceeds echoing strikes once again (with BiS slots it becomes blood boil > fire > echoing >>> abyss)
+fire skills work with ring of fire, flame wave, apocalypse & blood boil. They don't work with summon tainted, unlike how they work with summon fire golem

Tyvm for the explination. I havent fooled around with blood boil. I respeced for apocolypse to hopefully run hell cows a bit faster. If it doesnt work I guess im buying a token XD

I don't know how blood boil holds up for undergeared / mid gear. I know its the best build once you're fully geared, and I know nova->echoing when you're going from norm/nm -> hell with barebones insight