You're right about most of the stuff you said, except this:
: you cannot have two objects in the same x and z positions (at different y
: positions) at the same time visible on screen.
Unlike Doom, for example, you can still have one player, monster, etc. directly above another, like if a player in a netgame jumped from a ledge and passed over another player. Doom would not allow this, and you could actually trap people in narrow elevated places because the engine blocked movement. This is also why they programmed teleporters to kill a player if he was in the way when another player teleported into the same place.
Objects aren't a problem, but map architecture is. Just a little clarification!