You can cast Orzhov Charm on Doomed Traveler after you got the spirit token, or on Diregraf Ghoul to make you able to re-cast Gravecrawler in a pinned down - situation... ...the card is also good for saving your creatures from removal... ...about 40% of the time, I cast it for it's unconditional removal ability, though it costs me some life.......
For Disciple of Bolas, the Messenger is a perfect sacrifice, expecially if the Messenger attacked that turn with Knight of Infamy on board... ...same goes to Gravecrawler...
With the Angel, you can blink the Messenger & reset him, or blink Obzedat, may be Disciple of Bolas to make it trigger again... ...she's also good for saving your creatures as well...
In the sideboard, I have Elixir of Immortality for mill decks, expecially against Memory Adept... ...& it's also good when I need a little life boost against fast aggro decks, but I never use it, unless the opponent playes some kind of mill... ...some america aggro decks has Jace in the sideboard wich could be deadly...
Tormod's Crypt is for other grave decks...
Cartel Aristocrat is awesome for this deck, expecially when the opponent playes wipes... ...it's also an infinate blocker(as long as you have some to sacrifice)... ...with Doomed Traveler & Gravecrawler, it's amazing... ...though I don't need it in most of situations, it's still great for many... ...originally, I wanted to have it mainboard, but I realized how fast this deck could be without it, so I put it in the sideboard for times I need her... ...about every 3rd game, I need to put it in.......
Oblivion Ring is for various threats, that I can't get rid of with other stuff I have... ...like Runecaster's Pike, Detention Sphere, other Oblivion Ring(s), Gravedigger's Cage, Rest in Peace(even though it triggered, it's bothersome to have it still on the battlefield), etc....
Ultimate Price & Victim of Night... ...in case I need more/fast removal... ...there are some times I can use both & some times just 1 of them... ...so I need them both in the sideboard.......
...the extra Sever is very important, as well as the other one in the main deck... ...it can deal with tokens, I can cast it twice, indestructible/regenerationable stuff won't stop it, like Boros Charm, Lotleth Troll, etc... ...it's unconditional & can get rid of for example 2 Huntmaster at the same time (players like to have more on board, so they would transform togeather for more effectivity in their triggered ability)...
About the lands: I play 22 lands... ...not counting Cavern, I have 14 that could add black mana & 13 white... ...there are 18 lands that I could cast Messenger with & 4 that I couldn't... ...I usually use Cavern for zombie, but if I already have 3 black, I like to name spirit for Obzedat, or angel for Restoration Angel, sometimes human for the Aristocrat, or Disciple of Bolas (I have 4 Cavern, so it's not a problem)...