Your first point(about minm dmg drop) is valid and I wondered this as well. It seems however that pdf in general is ignored by the mirror dungeon mobs, I have no idea what their minimum value is set at but it would surely be worth it to test it with some nice min. dd set.
As for the SD, I've tested this in sunken and it used to lower the damage you received by a lot. This is however quite awhile ago and multiple patches have been applied scince (SD used to drop Invenom dmg too but this was changed). I'd not be suprised if this was changed since well...#Koreans