Spoilers for DRK chain and 3.0 MSQ.
Ishgard is populated by Elezen primarily, followed by Hyur. Either would be fine - your DRK class trainer, Fray Myste, is based on a genuine living Ishgardian Hyur Dark Knight. And the previous Azure Dragoon of Ishgard - premier among its formidable military - was a Hyur, too (Ser Alberic Bale). So although there are no Hyuran members of the High Houses, it's still possible for a Hyur to obtain, retain, and... lose... a high standing in Ishgardian society, especially via the avenue of becoming a military knight.
It's also worth noting that Ishgardian Elezen, ICly speaking, tend not to fall into categories of "Wildwood" or "Duskwight". Those are divisions that arose specifically from the unique living conditions in the Black Shroud, and they don't apply elsewhere. So Ishgardian Elezen are their own sub-race, with their own distinct and unique sub-racial trait (the blood of the great wyrm Ratatoskr flows through their veins, allowing them to transform into a dragon-like creature when fed dragonblood that was poured with great magick)...
...but since there's no "Ishgardian Elezen" character creator, it's each player's prerogative on what clan model they want to use to represent their character. FWIW, the majority of Ishgardian Elezen NPCs are on the Wildwood model.
ANYWAY.
I think everything else checks out. Nothing's screaming "that wouldn't work" to me. It's more likely that his darkside would awaken after he'd already received a soul crystal to allow it to, but it's not impossible for it to happen on its own, especially under such horrific circumstances (the Dark Arts originated from someone, right?).
It sounds like you already know a fair chunk of DRK lore, so this is more for the benefit of other people who might be reading the thread, but here's my write-up of all the DRK info I could find from the lore book.
Good luck!
Ishgard is populated by Elezen primarily, followed by Hyur. Either would be fine - your DRK class trainer, Fray Myste, is based on a genuine living Ishgardian Hyur Dark Knight. And the previous Azure Dragoon of Ishgard - premier among its formidable military - was a Hyur, too (Ser Alberic Bale). So although there are no Hyuran members of the High Houses, it's still possible for a Hyur to obtain, retain, and... lose... a high standing in Ishgardian society, especially via the avenue of becoming a military knight.
It's also worth noting that Ishgardian Elezen, ICly speaking, tend not to fall into categories of "Wildwood" or "Duskwight". Those are divisions that arose specifically from the unique living conditions in the Black Shroud, and they don't apply elsewhere. So Ishgardian Elezen are their own sub-race, with their own distinct and unique sub-racial trait (the blood of the great wyrm Ratatoskr flows through their veins, allowing them to transform into a dragon-like creature when fed dragonblood that was poured with great magick)...
...but since there's no "Ishgardian Elezen" character creator, it's each player's prerogative on what clan model they want to use to represent their character. FWIW, the majority of Ishgardian Elezen NPCs are on the Wildwood model.
ANYWAY.
I think everything else checks out. Nothing's screaming "that wouldn't work" to me. It's more likely that his darkside would awaken after he'd already received a soul crystal to allow it to, but it's not impossible for it to happen on its own, especially under such horrific circumstances (the Dark Arts originated from someone, right?).
It sounds like you already know a fair chunk of DRK lore, so this is more for the benefit of other people who might be reading the thread, but here's my write-up of all the DRK info I could find from the lore book.
Good luck!