Just going to compile the hypothesis (maybe theory) and try to answer any other questions here...

The Ardor
-A calamitous event on the Source that Rejoins a Reflection to it. By creating a massive aetheric disturbance or deficit on the Source, the dimensional barriers separating a Reflection from the Source are broken down, converting that Reflection into raw aether intended to fill the deficit and repair the aetheric flow. This aetheric flow is redirected to Zodiark, while Hydaelyn needs time to recuperate and civilization needs time to rebuild, creating the cycle of Astral and Umbral Eras. (Most recent Umbral Era very short because the disturbance was undone by Louisoix returning the aether from the summoning of the Twelve to the land, and civilization survived to continue creating the chaotic conditions that support Ardor.)

Balance
-Needs to be maintained on Reflections or they will be drowned in a Flood of Light or Flood of Dark. Neither is beneficial to the Ascians, as an imbalance does not produce an optimal amount of aether. There may also be the matter of a Reflection being superimposed on the Source during a Rejoining in the style of Terra and Gaia in IX, and a Void superimposing itself on the Source would make for less than ideal circumstances to initiate another Rejoining (possibly ever). Light and Dark in a certain balance produces the greatest amount of raw aether upon a Rejoining, which is then directed to Zodiark.

Primals
-Thoughtforms given physical manifestations by aether. Exist solely to drain Hydaelyn of aether, thus weakening her and making it ever more difficult to prevent further Rejoinings. Primals warp the flow of and devour aether, preventing it from being returned to the land, which slowly kills Hydaelyn and weakens her until then. This makes it much easier for the Ascians to initiate Rejoinings.

Zodiark and Hydaelyn
-God and Goddess of Dark and Light, respectively. Though associated with those elements, the elements themselves are not inherently good or evil. Heavensward has beat into our heads that it doesn't matter whether it's Light or Dark, it's how you use it that's important. Dark Knights use the power of darkness, but are still good people at heart. The Knights of the Round used the power of light, but intended to put the world under their heel. The Warriors of Darkness vanquished all the Ascians on the First Shard, only to have it start getting erased in a Flood of Light. The Thirteenth Reflection was drowned by Darkness, but that isn't beneficial to the Ascians' goals.

It's not so much that "Light and order are good because they are Hydaelyn's domain" or that "Darkness and chaos are bad because they are Zodiark's domain," but that "Hydaelyn is a benevolent goddess whose domain happens to be Light and Order" and "Zodiark is a covetous god whose domain happens to be Darkness and Chaos." (This would explain the Ascians' association with the Void despite it being useless for the purposes of a Rejoining.) Hypothetically one could reverse the roles with Zodiark as the benevolent Fathercrystal whose domain is still Darkness and Chaos and Hydaelyn as a tyrannical, banished goddess whose domain is still Light and Order... but I digress.

... well, that's the best hypothesis I can hash out with what information we currently have. I'm going to jump off the crazy train and wait for it to plow into my house of cards when we get another piece of information that contradicts it, lol. (And Zodiark is totes the moon! I called it a long time ago!)