Ranger powers requiring wielding two weapons don't have 'off-hand weapon' as a pre-requisite.
Example: Dire Wolverine Strike.
If this armor qualifies as a weapon for all powers, then wearing it means wearing a weapon, as well as other weapons you carry. This + Mordenkrad = Two weapons.
I'll count it again.
This. 1. Mordenkrad. 2.
Two weapons. Done.
Unless the power requires a weapon in the off-hand, explicitly, it doesn't ask where the weapon is equipped.
And if this armor is allowed with leather, it allows rogues to use a rapier without a proficiency slot.
No, sir, I don't think this is anything but a basic melee attack.
Besides, you have to factor in 'This is a basic melee attack' into everything you try to use it with.
Example: You want to use it to Booming Blade. Is Booming Blade a basic melee attack? Either the answer is 'No' because the property has that restriction, or yes, the property -makes it a basic melee attack- which means you can, just by wearing the armor, use Booming Blade with AOs.