"However I don't believe a unit of warriors in reserve can teleport through a deep striking monolith because neither of them are on the board when rolling for reserves."
There might be a slight loophole to this one wherein the situation occurs where the Necron player has decided to keep surplus troops in reserve as per the reserves special rule under necron warriors and rolled to have both come in on the same turn. When they arrive Warriors from reserve must emerge from a monolith portal, if no monolith when the units become available, they must wait until one is available."
It doesn't actually say that they can't both come in on the same turn. I guess a situation could exist where the player could roll for the Lith, deepstrike it, then roll for the warriors to become eligible from reserve and port them in during the movement phase as per the Lith rules in the codex. I could be totatlly wrong of course.