Heartzertauren can currently be found in the Chamber of Heart, and currently is selling items to teach you up to rank 3 of all of the new Azerite Essences. The Rise of Azshara Essences are also available for sale here.
While these new Essences should be functional, most visual effects on these new Essences are temporary, please keep that in mind with any feedback you may have. They will be getting new visual effects in a future PTR build.
This achievement will only require you to get rank 4 of all of a role’s Essences that were released in the Rise of Azshara update (patch 8.2). It should not become any more difficult to do with the addition of new Essences in this new patch.
This is currently working as intended. This cooldown reduction doesn’t apply to any of the “passive” Essences, including Vision of Perfection, Conflict and Strife (and any ability gained from slotting it), and the new Touch of the Everlasting.
The Formless Void mostly exists for the purposes of flexibility, which is why all roles can use it. It requires a lot of planning, it requires you to coordinate and know who you plan on borrowing what Essence from, and for what situation, and make sure that the cooldown incurred doesn’t interfere with whatever the next situation you have to manage is.
As a note, you can also borrow something from a player of the enemy faction. The targeting is very unrestricted other than the obvious - it has to be a player, they need to have an Essence currently slotted that is valid to copy (fully passives Major Essences are ineligible).
You can borrow any eligible Essence, even if it is not normally available to your current role. For example, a Protection Paladin player can borrow a healer specific thing such as the new Spirit of Preservation, or a DPS specific thing such as Essence of the Focusing Iris.
As a note for Mistweaver monks, a bug was recently fixed with regards to Spirit of Preservation’s minor Essence effect and how it interacts with Vivify.
Prior to this bug fix, both the increased healing amount and critical strike chance from the minor Essence applied to all healing effects of Vivify, not just the primary target healing. This wasn’t intended. The essence should only be affecting the primary Vivify target’s healing value and crit chance, similar to how it works with something like Flash Heal or Healing Surge. A future PTR build should have this bug fix in it.