You're both correct - a story is a story. And so long as Helseth approves he might not have you assasinated for writing it

... ummmm, let me rephrase that: Helseth might have you assassiated if he approves of the story just in case you ever decide to write a retraction.
However I just wanted to say that there is a murky side to Helseth's background that goes beyond what is written in the game books - including the book about the Ebony dagger & his father (the Dark Elf Emperor was he?). And that I witnessed the dark side myself
That does not mean I think you have to include it in your story, just wanted you to be aware of what many people believe and know of him.
It does occur that Helsseth may have been 'sheltered' in some way from the environment in which the Underking lives during his fostering and that might be fun to explore ... I just have the feeling that it is the Underking who would need to be sheltered from Helseth.