1) Usually only 1 level down or the same level, except if the level specifies a DesiredMonsterSpawn (then it is -4 to +2). This is all controlled by MinMonsterLevelOffset, MaxMonsterLevelOffset, MinDesiredMonsterLevelOffset, and MaxDesiredMonsterLevelOffset in world.gdb.
2) The starting point switches at level 25 but a normal difficulty world can get to level 30 at the deepest part of the dungeon.
3) In theory it should work, like you said the Totems work like that.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, Din's Legacy, & Drox Operative 2