I had always assumed that the factor from Darkness stacked with the factor from Dim Light. If that’s the case, then you’re looking an an increase in obstacle of 2, which is pretty statistically punishing. But I might have that wrong.
The more I think about it the penalties for darkness seem right.
I’d maybe make navigating to a previously mapped room an obstacle in the dark however … and up the factor by one for every extra room they need to go. This would make getting back out either slow or very treacherous! Failure would result in some good twists I think (henchman with the lootbag goes missing, etc.)