As good as Liliana of the Veil is, she is not how we win games (see the last three sections for that). Especially so in a multiplayer game, she is prone to damage, direct attacks, and the occasional Hero's Downfall. I have never once gotten her high enough to use her ultimate, although I've never needed it. Most of the value we get out of her before she goes back to the command zone are in her first two abilities. We can guarantee either a card or a creature gone each turn until she leaves the field.
This deck is not creature heavy at all, having just six, seven if including Marit Lage. It is essentially guaranteed that she will return to the command zone at least once. This should not bother us, however, because, as said before, we do not rely on her to win the game. Instead, she is a step in the process to winning the game.
Plan on casting LotV twice in a game: once early on, during turns 2-4, then much later on to help close off the game.
Dark Petition may seem like a peculiar choice as a signature spell. Why not use one of the many discard spells? The way this deck plays, there will basically always be a discard effect either in your hand or within easy reach. It becomes overly redundant to always have one at the ready. Your opponent's hands will drain too fast as a result. Commander tax also applying to a signature spell also means the more times you cast it, the less value you get for the increasing cost. There is also the fact that we always need Liliana of the Veil in order to cast it. Because we do not depend on her to win, this means we have spell out of reach for the majority of the game.
Rather, the signature spell should be something we don't always need, but when we do need it, it has a big effect. Dark Petition's rangeless tutor effect is already great, but the extra BBB means we have a fair shot of immediately casting whatever we searched for. Spell Mastery will always be enabled, as the vast majority of deck is instants and sorceries.