(A) But that coexistence happened because both host and Fallen were compatible. I'd like to think I wouldn't be as nuts as Deirde, making me incompatible with said Coin.
(B) Immortality seems to be a standard side effect of all Coins. Quintus Cassius really showed his age once Saluriel was taken from him, and considering the ages of Nic and Tessa, among others, the trait seems ubiquitous. Ditto Hellfire. If you're a magic user, then you'd have that with any Coin that doesn't steamroll you into a husk. The armor and tensile weapons are unique to her, but I'd be more interested in learning magic.
(A) Unlike others, I'm assuming I'm going to be a bad guy when taking up a coin. I'm going to do despicable things, to some degree or another. But I'd bargain the shared agenda. For every big evil plot, I get to met out some Griff-style justice to those I deem deserving. Which would presumably add to the fearful chaos that Denarians draw power from.
(B) Immortality, yes. It's just one of the benefits in general, along with hellfire. But I'm not presuming that the other Denarians can bestow magical ability when it doesn't pre-exist. I figure Namshiel prefers hosts with magical ability. So (potentially wrongfully) assuming that, then we already have some magical ability. That means I don't *need* the Denarians to teach me; I just need to learn how. Or I work with one of the Denarians that *can* teach magic. Just talk to Namshiel's host, and in return for some pointers, I'll help him raid a village or something.