[Spoiler] Isn't Dark Knight using Akasa ?

[Lyth]

Sanderson's first law is not 'Thou shalt not introduce in new world-building lore in the sequels of a serial work.' It's a principle of foreshadowing, in which you plant plot elements well in advance of them becoming plot significant so that the reader/viewer gets an 'aha' moment when you stab them with a plot twist. (Otherwise you'd know the complete ideals of all the Knights Radiant by now and not just be fed them during climactic plot moments, but I digress.)

On the subject of Limit Breaks, there's no clear cut line on what exactly is Aether and what is Dynamis. Emet, for example, uses the line 'Mine is the Aether!' when he uses a Caster Limit Break. Likewise, it's never definitively stated what Elidibus uses to power his Limit Breaks. There is, however, this unresolved bit of dialogue from your showdown with him (Hope's Confluence):

What of it?! I have my mission! I am Elidibus! And it is my duty to steer mankind and the very star upon their true course. This I swore to... to someone. We spoke, and I swore... what? What did I...?

Ah, poor old amnesic Elidibus. I really can't help but wonder if his Warrior of Light, his summoning magicks, and his 'Limit Breaks' all draw inspiration from a certain chance encounter on Elpis, countless years before. I'll laugh if he makes that promise after discovering the cause of the terrible cry from within the earth that set off the Final Days. Perhaps in a place in the depths with lots of Creation magic.

So now let's look at the A12 example that you put out. If Hermes is to be believed, the unsundered Amaurotians were too aetherically dense to perceive, manipulate, or interact with Dynamis in a meaningful way outside a few observations made about entelechies. So how did the Final Days as a Dynamis based phenomenon have any effect on the Ancients? It affected their creation magic. And what is the modern day derivative of creation magic? Primal summoning. Meaning that if Meteion can warp an Ancient's creation magicks on Etheirys from the edge of the universe, then it wouldn't be a stretch for us to unconsciously use Dynamis inside a primal.

Part of the difficulty here is that we don't have the rules, so there probably isn't enough here for you to go off of show a contradiction just yet. Can you utilize Dynamis miles beneath the veil of Zodiark's moving celestial Aether currents? It would seem that way, otherwise the Elpis flower from Sharlayan wouldn't change colour in the present era. Does Dynamis have a mass-like property similar to Aether, or is it completely intangible? What are the Aether thresholds at which you can use/not use Dynamis effectively? If the Garleans are unable to manipulate aether but their Reapers are capable of summoning Voidsent, does that mean that Void magic is based off of Dynamis? Are there any civilizations that are actually based off of utilizing Dynamis (the Amaurotian quest Debate and Discourse, for example, alludes to another civilization co-existing at the same time as the Ancients 'across the pond' that had the capabilities to build a metropolis and a calamity of their own to deal with.) As you can see, this doesn't just have implications for Endwalker, but future storytelling as well.

[Veloran]

Sanderson's first law is not 'Thou shalt not introduce in new world-building lore in the sequels of a serial work.' It's a principle of foreshadowing, in which you plant plot elements well in advance of them becoming plot significant so that the reader/viewer gets an 'aha' moment when you stab them with a plot twist.

Yes, but as Sanderson says he had already foreshadowed the existence of a secondary magic system in the first book, but still recognized that introducing it at the very end of that book to resolve the plot was erroneous writing. In the context of XIV, this is where I think the "missing expansion" between Shadowbringers and Endwalker, the original 6.0, would have significantly smoothed out the story. Introducing Dynamis there and allowing us to stew and learn more about it for a couple of years would have integrated it much more seamlessly into the narrative before we went into the final chapter.

On the subject of Limit Breaks, there's no clear cut line on what exactly is Aether and what is Dynamis. Emet, for example, uses the line 'Mine is the Aether!' when he uses a Caster Limit Break. Likewise, it's never definitively stated what Elidibus uses to power his Limit Breaks.

As you point you, we lack the rules about any of this, and so are left with nothing but headcanon to fill the blanks. For example, all player Limit Breaks are observed as aetherical by the characters in-universe through the EE, which would suggest that, if LBs are indeed partly Dynamis, that both aether and Dynamis are combining to produce the effect. Which runs in contradiction to the notion that they are incompatible.

So how did the Final Days as a Dynamis based phenomenon have any effect on the Ancients? It affected their creation magic. And what is the modern day derivative of creation magic? Primal summoning. Meaning that if Meteion can warp an Ancient's creation magicks on Etheirys from the edge of the universe, then it wouldn't be a stretch for us to unconsciously use Dynamis inside a primal.

As above, there is no explanation. We don't know how the Ancient's magicks were warped, only that concepts were seemingly siphoned from their minds and made real by creation magics taken from their control. We have no idea how Dynamis could be capable of this. And with the issue of the Primals, there's also the fact that they are formed partially through faith and prayer - Something that, now, we could easily conceptualize as Dynamis. But if so, that raises the question of how Hydaelyn and Zodiark could even exist, given the Ancients evident inability to manipulate Dynamis.

While I am interested in the ways Dynamis could factor into the story in the future, at the moment I have my doubts as to the question of whether or not they'll get around to explaining some of the oddities surrounding it, such as what is and is not Dynamis on a basic level. To do so in-game we might need to return to ARR-levels of exposition dumps. Otherwise we'll be left awaiting a potential new EE in the future.