I question how useful the Maskwood Nexus effects are when most of your creatures are already horrors. Is it worth dedicating several slots in your deck and an entire early turn to cards that will affect one or two cards in most games?

I'd say you're better off cutting those and any cards that are only good if you get one of the aforementioned Nexus effects for more interaction. Whether you're going for more mill or trying to win with an army of 3/1's you really need to be able to deal with the game-ending threats that will arise while you're setting up. Might I recommend the new Singularity Rupture?

And for any mill deck you should definitely be running Breach the Multiverse to snatch the creatures you'll be making people mill. It nets you the four best creatures for seven mana, which is never a bad deal. Plus it makes all your creatures Phyrexian's, which means that if you have your commander out they all share a creature type for your background.