Where are all his dialogues?

All the theory section has essentially been debunked with SotFS, I see. Should be edited.

Aldia is not Seath lol. Seath possessed the Duke, created Freja, the scorpion hybrids, and other abominations, and his soul is found at Freja’s domain. Aldia, furthermore, sought to break the cycle of Fire and Dark, to use and even combine them in an effort to unveil the true nature of existence, and mankind’s place in it, and potentially learn to mitigate or avoid the downsides of the undead “curse”. Aldia would hold no love at all for Seath, who he would view as complicit in Gwyn’s sins.

So...where is Aldia's soul? Did he stash it in another vessel? Did it just dissipate into nothing? Was losing it required to exist outside the cycle?

Idk why there's a whole section on Aldia being the Duke when this guy exists:
https://darksouls2.wiki.fextralife.com/Duke+Tseldora

If you killed Nashandra and Aldia didn't appear then go kill vendrick and use a bonfire ascetic on kings gate bonfire in drangleic castle and then when you kill watcher/defender and Nashandra again Aldia will appear you do Not need to go into the next NG cycle if you missed him.

bruh doesn't even drop souls

Aldia is definitely not the Duke of bright stone cove. It's literally the hollow guy with the rapier after Freja. Freja is likely the reincarnation of Seath as she drops his soul, though it could be the dead dragon, or the duke.

If Aldia was the Duke, then they would have just said Aldia, like they do everywhere else.

"In order to make this boss accessible, his dialogue must be exhausted at all three places he appears." Does it need to be in order?

Honestly not as bad as people claim. Yea yea he doesn't drop **** yada yada like these 150k souls would make any kind of diffrence in the endgame maybe like max 4+ to some stat or 2 DLC armor sets for which DLC provides souls anyway. Great OST cool looks and def better than nashandra which is not an achievement but still a better final boss than her.

I just saw Vengarl's summon sign and I definitely did not kill his body this playthrough, so that is also incorrect.