Played that deck for awhile. It's pretty fun and quite competitive with the right SB choices. I've played all the cards you're suggesting, and tested others. My thoughts are... Abrupt Decay is obviously great, but go for the throat and dismember could/should be x4 victim of night (especially since you mentioned wanting to beat affinity). x4 Phyrexian Arena always felt like one too many. I'd probably go for 3. You also want x4 gatekeeper of malakir. Edict effect+devotion is just too good for this deck. Gatekeeper also replaces liliana. In fact, I ended up not running her at all, mostly because you don't want to be discarding cards in this deck and gatekeeper was SO good. In addition to drown in sorrow, you should also think about ratchet bomb. It's great against affinity, bogles, and junk. The worst part about this deck is the burn matchup. If you think you can rely on Gray Merchant to save you, you are wrong. The only answer that ever worked well was x4 chalice in the sideboard and some wrench minds. Lastly, you might consider dropping the green splash altogether. I did that, and the deck didn't really get any worse, and it helped the burn matchup a lot. I just used victim of night, gatekeeper, and geth's verdict as the removal. Not losing life off of the fetches made thoughtseize better... something to think about. You mentioned twin, and all the hand disruption I ran was enough to eat that deck for breakfast usually. Affinity matchup is a little better with the green splash, but like I said, ratchet bombs, chalice, and a couple main deck drown in sorrow made it downright winnable. Regarding arena, I played around with prey upon and pit fight, and that was fun, not sure if it was really all that good. Vampire nighthawk is OK, best thing about him is that he draws away bolts from your face. Lastly, some other great devotion oriented cards to think about are Geralf's Messenger, Bloodghast, a singleton Whip of Erebos (if you run enough creatures), Vampire Hexmage in the SB if you face lots of decks with PWs, and scepter of fugue. If you go mono black, you can also run a couple cavern of souls so that you can stick the obliterator and the merchant.

That's all I can think of off the top of my head. Feel free to ask more. See you in Vegas!