One thing not even explained in the website is whether or not Xander was kidnapped, the lyrics clearly state that he followed his father to Heaven's Cove so why was he worried about his life? And why did Daryus threaten him with his son's life?
Well, the way I see it, had Xander been kidnapped by Daryus, Arhys wouldn't have stayed all night in his home (which the track description for "A Tempting Offer" makes me believe he did). Knowing who Arhys is, I guess he would have chased Daryus and get his son back. So, in my opinion: no, he wasn't kidnapped.
Regarding the second question: I don't think Daryus threatened Arhys with Xander's life. From what I've read, Daryus offer was:
A. for the Empire to have Gabriel and for Xander to have a better life, or
B. for the Empire to try having Gabriel and for Xander to remain as is,
Daryus
did threaten to kill Xander, but that was once Arhys was dead and Xander appeared out of nowhere surprising the Prince. I guess he threaten to kill him so there'd be no witness to the crime. He would have had to act fast to cover up - I think that there still was time
Anyway, that's all speculation, and maybe I'm forgetting a bit of text, so be sure to correct me if I'm missing something!