Also, quick question: how can I justify Cadash having a British accent? I think Alix's tone would suit this chara better than Sumalee's, but EVERY dwarf in this series, even lifelong surfacers like Harding and Varric, have American accents.
I think the only exception is Bodahn. So, has anybody else thought of a decent headcanon for this? 
Hmm, perhaps it could be something similar to my headcanon for the origin of Bodahn's pseudo-English accent?
My pet theory for why Bodahn is the only Englishman in Orzammar (despite being played by an American) is that he originally affected his accent to make himself sound more posh and cultured, thinking it helped when dealing with the nobles visiting his shop in Orzammar? He kept it up even after becoming a travelling merchant on the surface, as well as being perfect for the role of Hawke's butler in DA2? Although, after a decade of using it, I think that it's more or less stuck and become his natural speaking voice?
Could be that Cadash sounds British because they're similarly putting the accent on, although as Gilsa pointed out, their reasons could be far more insidious and manipulative than Bodahn's attempt at culture? Cadash recognises that sharing the same accent as someone else often puts people more at ease, so by affecting an accent that sounds vaguely Ferelden or Marcher, they were able to subtly manipulate people they dealt with as part of their work in the Carta to their advantage, whether it be to lower their guard or simply help make business deals?
Perhaps they used it as part of their disguise when going undercover to spy on the Conclave and simply chose to maintain the accent when everything hit the fan and they were caught in the blast? After all, it wouldn't exactly build trust with Leliana or Cassandra to reveal they've been using a fake accent, especially right after you've just convinced them that you're not the one who caused the Breach?
(Actually, this might also work as well to explain why Lavellan is one of the few Dalish these days without a Welsh or Irish accent... even though it'd not exactly be a good disguise, what with the vallaslin all over their face?)