Continuity Errors

The thing about Hilwen Mines and DGs is not a continuity error. The discovery of the mines and engineering stuff by giants may have been recent, but they've been there since before the elf/giant wars.

The episode 4/5 cutscene thing might not be an error. Ingame, all the changes to Iria just happened in the update but, iirc, npcs talk about these changes as if they're something happening over time; suddenly, but not all at once. The npc conversations I've read so far haven't mentioned the changes to the towns themselves yet, so, as far as the story is concerned, the town remodeling hasn't happened yet.

When did Cai come in contact with Soul Stream?

Where does it say that Tuatha de Danann are children of the Partholon?