I didn’t read the comment section, so apologize if someone else has already answered these 
Does the Attack Speed being the best stat after 20% Crit Chance rule still apply when considering WHC’s Passive?
Heavily depends on difficulty since Killing Shot is tied to enemy max health, but in general unless you have range breakpoints to reach just run AS…
Eternal Guard Block Cost interaction?
Weapon Block Cost/property Block Cost Reduction has nothing to do with Eternal Guard. If the attack stamina damage base cost is <2, Eternal Gard works. You can check out Prismism’s block cost table/calculator(for cata) for the enemy attack stamina damage: Vermintide 2 Block Cost Calculator (Cata) - Google Sheets
Witch Hunt stacking interaction?
Currently it doesn’t stack with Huntsman Make 'Em Bleed, Bounty Hunter Open Wounds, melee trait Off Balance
CDR in seconds Double-Shotted?
28 sec per headshot, max 56 sec. Pre-SOTT patch you can get 100% CDR by landing 3 headshots.
Zealot Career Skill:
IMO it’s worth mentioning after using career skill/during career skill AS buff, Zealot can walk through enemies. This is very good for picking up elites inside horde.
Is Righteous Fury tied to damage Cleave, or does Stagger-Cleave trigger the damage effect too?
Only works on units that you damaged with melee weapon, so if you only cleave them with stagger cleave it does nothing.