But one says that "Damage from an attack or from a source that doesn’t require an attack roll (such as the paladin’s divine challenge or the fighter’s cleave) destroys a minion." Effects don't require attack rolls ; as in, you can use them without making any attack roll. This contradicts your interpretation of the "missed attack never damages a minion."
You're considering Cloud of Daggers to contain an attack (a source of damage that requires an attack roll), and an effect (a source of damage that doesn't require an attack roll).
SadisticFishing considers Cloud of Daggers to be a source of damage that requires an attack roll, some of which applies on a hit, and some of which applies to creatures in the area.
Since, under his reading, the source of damage is the power, not the effect, both the "Hit" and the "Effect" are both consequences of Cloud of Daggers, a source of damage which requires an attack roll. The Effect, taken in isolation, might not require an attack roll, but the power does.
Once you decide that "Cloud of Daggers" is an attack, SadisticFishing's argument parses. It leads to bizarre and non-intuitive results, like Arbitrary's Stinking Cloud example, but
once we accept Cloud of Daggers as 'an attack', bizarre and non-intuitive doesn't make it wrong.
That's the crucial error. If SadisticFishing can be convinced that Cloud of Daggers is not 'an attack', but rather that Cloud of Daggers is a power that incorporates both an attack and an effect as discrete entities, everything else falls into place. But until he can be convinced of that one piece of the puzzle, everything else in his argument follows logically from the original incorrect assumption.
-Hyp.