Heavencast only affects the A1 :) The reason to take it for Husk is to improve his chances of landing the provoke on A1, which is quite useful - and also, the other blessings don't really do much for him, especially at low levels.
As for turning off the A3 - the reasoning there, I guess, is that on auto you'd want him to be using the A1 to maximize his chance of landing provoke. That said, I doubt your team would work well on auto to begin with, so that's a moot point - and his A3 will definitely hit harder than his A1, so if you don't need to land the provoke, you should definitely use the A3. The exception to this is if you are running Reflex, in which case you want to make sure the cooldown decrease goes to the A2.
In terms of build, I would just focus on maximizing HP/CRATE/CDMG, and trying to get his acc to a reasonable place. He shouldn't be your primary provoker, but it also doesn't hurt for him to be able to backup provoke.