I finally figured out the Mortis arc.
(I've watched this story at least four times, and it bothered me a little on most rewatches because it seems a lot of the interpretation surrounding this story involves the Light/Dark Sides following yin/yang principles, and this contributes to the (incorrect) idea that "balance" means a balance between numbers of Jedi and Sith. Thing is, that interpretation goes against what George Lucas has said about the Light/Dark Sides; and The Clone Wars was created as Lucas-based canon before the Disney buyout. It's one thing to change from the 2D version of the Clone Wars story to the cartoon version; it's another thing to rewrite the fundamentals of the Light and Dark Sides!)
So, here's how I now understand it.
Here's the thing: the Light and Dark Sides both exist. They always have existed that we know of, as far as we can tell they're always going to exist. That's a given. But people don't have to give in to the Dark Side and act on their darker impulses for balance to exist. The Dark Side itself, as George Lucas himself says, creates imbalance. Personally, I have framed it as the Light Side following the will of the Force - whether the Force wills life or death, gain or loss, etc. - while the Dark Side tries to wrest control away and bend the Force to its own ends.
And the Mortis arc actually depicts this.
✨ The story does indicate that in terms of actual light/dark, night/day, creation/destruction - all this must exist in balance. And the Force DOES keep all these factors in balance.
There is a distinction, however, between light/dark as natural forces of nature, versus the Light Side and Dark Side in terms of Force mythology. This is best seen by viewing the story in its LITERAL sense: the Father/Daughter/Son are powerful Force users who have gone into exile on Mortis. The Daughter tends toward life/creation, the Son tends toward death/destruction, the Father keeps a balance between the creation/destruction. They all, as Force users, can be tempted by the Dark Side of the Force just as any other Force wielder can be (indicated by the fact that the Son is spoken of several times as "giving himself over to the Dark Side," not that he is the Dark Side itself). There's also the fact that the Father/Son/Daughter all die, but the Force itself obviously doesn't die, so in the end these characters literally are what they say they are.
There are multiple layers to this story, of course, so there's more than just the literal to account for.
✨ If we are to take the Father/Son/Daughter as archetypes, we have the Father as a representation of the Force itself. The Daughter represents the Light Side, and the Son represents the Dark Side. Both the Daughter and the Son exist as the Light/Dark Sides exist, and the Father is the authority over both of them. Throughout most of the story, it is heavily implied that the Daughter follows the Father's lead (the Light Side seeks to align with the will of the Force). Meanwhile, the Son bucks against the Father's authority (the Dark Side tries to exert its own control) - and this creates imbalance.
The Dark Side is imbalance.
✨ It becomes even more interesting if we take the Mortis story as a depiction of the current struggle between the Jedi and the Sith. In this case, the Father, being the one who maintains balance, could be the Chosen One. The Daughter represents the Jedi, as they strive to adhere to the Light Side. The Son represents the Sith, as they act on the Dark Side. The Daughter strives to follow the will of the Force; but when the Son becomes more and more of a threat, the Daughter makes a critical mistake by doing what has been forbidden - namely, releasing the dagger - to kill the Son, and ends up losing her own life. The Father doesn't succumb to the Dark Side himself, unlike Anakin; but, like Anakin, he does keep looking to others to restore the balance. Ultimately, however, realizing the Son is out of control, the Father sacrifices his own life and takes the Son with him. In a similar fashion, throughout the millennia, the Jedi - by striving to act on the Light Side - seek to follow the will of the Force. But when the Sith become increasingly powerful and practically back the Jedi into a corner, the Jedi end up making critical mistakes and resorting to previously unthinkable methods to try to restore balance, and that ultimately results in their downfall. And while various attempts by others to fight back against the darkness are important, in the end, Anakin (the Chosen One) has to sacrifice himself and allow himself to die in order to end the Sith and restore balance.
So anyway, those are my takes on the Mortis arc now. I firmly believe Lucas's views on the Dark Side itself creating imbalance remain intact. The Dark Side exists and the temptation to fall to the Dark Side is always there; but balance in the Force is NOT achieved by people acting on the Dark Side and/or wiping out the Light Side adherents to be "equal to" the Dark. Rather, balance is achieved by the Force itself when people strive to follow the Light - namely, by acting selflessly, letting go of possessiveness, and letting the will of the Force take the lead.
