Have I written myself into a plot hole?

[valadil]

Seems like something he'd like to have anyhow, though. Perhaps even if the 4ed version doesn't have it innately, the doppelganger has had some sort of magic item crafted that grants it to him? It'd be an extremely practical investment if he's into the impersonate-a-life business, for situations exactly like this.

What's the doppelganger's temperament like? Is he an evil machinator, who took the noble's life and position to set nefarious plans in motion? Did he take the noble's life just so he could live the rich life? Or is he actually a fairly decent guy who's impersonating the dead noble for less ignoble ends? That could make a big difference in how he'd deal with inquisitive PCs.

Agreed. I see no reason why he wouldn't have a stack of detect throught ritual scrolls sitting next to him at all times.

Definitely an evil machinator. He doesn't have too many overly circuitous long term plots in motion. The noble he killed was a political enemy for all intents and purposes, so he's trying to shift things in his own favor.