Interesting Although I can't stress enough about Chaos as Chaos itself is not a demonic being but merely the opposite of Order. I feel that the final boss you face in AoS is not Chaos but a creature born out of Chaos itself. Also if you look at it the creature itself is also very orderly in nature as well, and there is no order in raw Chaos whatsoever. Amatsu-Mikaboshi (as you mentioned) is the primordial god of Chaos and the stars, but he is not Chaos itself.
In fact, in the texts I have researched, it seems that he's one and the same with chaos as the primordial force itself, formless and sometimes called "Amenokagaseo", while his personification as the "god with power over chaos" happens in the Ashihara-no-Nakatsukuni-Heitei segment of the Nihon Shoki, and he's called "Amatsumikaboshi" there and is "physically" defeated by Takehazuchi no Mikoto. He's also explicitly referred to as "evil" and "rebel" in this segment, but when going by other names, he's not. I'm going with "Mikaboshi" to simplify things for everyone.
Just as you mentioned, Mikaboshi is the antithesis of order and life, and not exactly evil. When In and Yo came into existence, Mikaboshi's reign came to an end, and his chaos was shattered, but not erased. His influence would come to exist in humans, and would push them to give in to their chaotic desires.
These texts are not very clear, and Mikaboshi tends to be personified or not as convenience demands. He's both the primordial force that drives humanity's passion/instincts, and a stargod who rebels against the Shinto gods born from Izanagi. There is also the issue that, as time passed and interpretations were made, he was bundled together with a bunch of other entities, evil or not, changing names as it went. I'm sorry I failed to be more clear about this aspect. All I just said, btw, is not me inventing things for Castlevania, this is actually how the lore goes.
I'm going with two different interpretations of this that are not still set in stone, so you guys can debate which is more fitting. Either one is fine by me:
1. Chaos is the distorted, identity-less form of Mikaboshi after being subjugated by Dracula.
2. Chaos is the concept which Mikaboshi rules over. Mikaboshi, in turn, is the entity possessing the Demon Castle's walls. Dracula rules over him, and therefore chaos does what Dracula wants through Mikaboshi.
I slightly dislike the 2th option because it generates another entity in the scheme of things when the 1st option only requires one entity. But, as I said, I've no issues with using it if you guys agree.
As for Chaos having a "recognizable" form, I think it has more to do with a videogame's symbolic representation than with the philosophical ramifications and consequences of "chaos having no shape." His first form is orderly, but his second form is far more abstract and chaotic.
Also about the demon castle. Liked the info you did for this one. However I think Dracula would have made Castlevania already before Trevor and Co. stormed the place. The Castle from CV III already contains the elements of an active creature of chaos so I do believe that everything you mentioned happened before Trevor arrives.
This part is slightly complicated.
I'm aware that the castle already contained a great power by CVIII, because Curse of Darkness mentions it all over. Trevor Belmont is able to seal its power using his blood, but the seal is broken by Hector and the castle is resurrected.
What I did here, however, is say that before Dracula's death, the castle was already magical, but was not alive by being possessed and did not have the power to resurrect, therefore allowing a simple blood seal to banish it. Once the seal was broken, however, Dracula's spiritual castle brought the physical castle back into existence (the spiritual castle being the one already possessed by Mikaboshi). Now the castle was impossible to seal with a simple blood seal like Trevor's, because it had been possessed by an entity that would not allow for it.
HOWEVER, the game's name is "Legend of the Demon Castle" after all, so yes, I'm trying to fit the story above on a plot BEFORE Trevor arrives. I'll just have to change the pact's terms slightly for it to make sense. I propose the following fix:
-Dracula makes the pact. Mikaboshi comes to Earth and posseses his castle, morphing it into a chaotic deathtrap and connecting it to the Demon Realm as a way to allow Dracula to summon demons by proxy inside it. The castle therefore becomes the "Demon Castle."
-Dracula is defeated by Trevor Belmont. Mikaboshi imediatelly grabs the soul offered in the pact (Walter's) thinking it belongs to Dracula.
-Mikaboshi notices the ruse and unpossesses the castle causing it to crumble, returning to the Demon Realm seeking retribution against Dracula.
-Dracula subdues Mikaboshi and turns him into his spiritual castle. Now Mikaboshi is forever doomed to be the Demon Castle, resurrecting the physical castle when Dracula returns, and crumbling it when Dracula is gone.
This solves the issue and makes the castle into the "demon castle" before Trevor arrives, while still maintaining the overall story.