How else does Hyadelyn watch everything going on in the Aetherial Sea and have full knowledge of what's happening? They just do. Plotwise it's not mentioned how they do it but they just have the ability to do so from the Lifestream. According to FF7, the Lifestream contains the essence of the planet and the memories, emotions, memory, knowledge of all who have lived on it. The Aetherial Sea/lifestream in FF14 has shown to be able to recollect memories that were thought to be lost in their lifetime. We also know people who teleport travel through the lifestream with a beacon to their destination after being reduced to an aetheric consciousness before arriving at their destination. Hyadelyn has the ability to drag a person's consciousness into the Aetherial Sea to communicate. In all likelihood, they were just naturally able to gather the information in the Aetherial Sea because the Aetherial Sea makes that information readily available for them to see.
The full quote from your examples:
Heavenly Yuki - Some cultures believe that when the dead are not provided proper burials, their souls remain trapped in the corporeal realm. Attuned to the scent of mortality, these tormented spirits seek out their neglected remains and “possess” them in much the same manner voidsent inhabit vessels to facilitate their entry into our plane, leading some scholars to draw the conclusion there may be some connection between the void and the afterlife.
Revenant Things FATE - Sometimes, people are unaware when the end knocks, and their souls miss the opportunity to journey back into the Lifestream. These wretched creatures are known as revenants, and they need your help in realizing there is no place for them in this realm.
Hunt: Flame Sergeant Dalvag - “Flame Sergeant Dalvag was among the thousands of defenders who met the Garlean invasion force in northern Thanalan soon after the Calamity. His company was ill placed to hold their position, yet given strict orders not to retreat." "Many fell, and suffered much in the falling. Multiple witnesses attest that Dalvag was the last of the company and fought as a man possessed till the end, the names of the dead tumbling in hoarse barks through cracked lips. Scholars theorize that his mortal remains are animated by a thirst for vengeance alone. Flame Sergeant Dalvag is classified as a Voidsent (Persona).
Hunt: Alben the Ashen - “When the Hyuran tribe came to Thanalan some eight hundred years ago, Albin the Ashen was at the head of one of the columns." "Then-native Belah'dians rose against the invaders and prevailed, as history records. Albin was but one of many slain in the abortive conquest, yet his mortal coil still roams the land in search of vengeance.” Alben the Ashen is classified as a Ashkin (Zombie).
Heavenly Yuki and Revenants have *corporeal forms*. Asahi is neither a Heavenly Yuki (he did not possess his own body as Fandaniel is inhabiting it) nor a Revenant (He is fully aware of his corpse was hijacked, so it's not as if he unaware he is dead).
As for Flame Sergeant Dalvag, his race is considered a Voidsent, specifically the race of the persona. Asahi is most assuredly not a voidsent.
We learn a bit more lore on what happens to Flame Sergeant Dalvag from Dalvag's Final Flame. Much is told of the corpse-soldier, Flame Sergeant Dalvag, yet mainstream scholarship has lagged woefully on the matter of what became of the rest of him.
Efforts by hunter-scholars with intimate knowledge of northern Thanalan have resulted in persuasive identification of a malevolent local demon as the late sergeant's spirit. This one's race is considered an Ashkin (Bhoot).
Ashkin represent beings which, at one time, were full to bursting with vigor, but, for some reason (most commonly death), have that life energy drained of them leaving naught but a shell—a shell that is then reanimated by some outside means, denying the creature's thoroughgoing return, be it either physical or spiritual, to the Lifestream. Ashkin are united by the characteristic that they cannot die in the conventional sense of the word. While the precise origins of many beings in this class are shrouded in mystery, naturalists theorize that many ashkin are created when souls are prevented from rejoining the Lifestream, causing them to take on grotesque forms and haunt the material plane in search of peace that never comes.
None of this applies to Asahi as he himself was not reanimated after having his life energy drained. His body is also not impervious to death, nor did his body take on a grotesque form.
Zombie - These undead soldiers are created through dark rites of forbidden spellcraft. Though slow and shambling, zombies are relentless and terrifying foes who know neither pain nor fear. The incredible feats achieved by the Immortal Flames of old gave rise to the rumor that the Grand Company's ranks harbored the living dead—a claim which the Ul'dahn royal palace declared unworthy of comment.
Asahi is neither an Ashkin or a zombie. He was not recreated as an undead with spellcraft.