Cloak reads, "The demon's Power is subtracted from the Perception dice of anyone who might perceive the user."

This is not invisibility, and that's not what it says. It's just a mechanical effect. It doesn't say anything about what the user is doing or what is not being perceived.

Does the defender of a Cloaked attacker incur a penalty? (cf. Invisibility.)

Nope. Once you attack, no Cloak. There's no true "invisibility" in Sorcerer. In all of the source literature and film, invisibility during combat is just a special effect; the opponent may look desperate and scared, but somehow he can always "see" the attack just in time to parry or fling himself aside. [1]

The "Passive Cloak" for Inconspicuous Demons is the only Ability that any demon type gets for free (the suggestion of getting Cover for Passer Demons is just that... a suggestion). "Passive" in this case means "does not count against Stamina as an ability-use." It is otherwise identical to the 'normal' Cloak ability.

1. If the user is just "around," which is to say present in the scene but not doing anything, it's treated as unnoticed by anyone.
2. If the user is doing something that isn't really in-your-face, but is nonetheless quite likely objectionable (i.e. conflict-of-interest), OR if 'the target is looking for the cloaked user or something like it, then it's a usual conflict roll, based on whether it gets spotted or not. This is treated as a normal conflict roll except the spottee gets the Cloak's Power in bonus dice (usually very effective). This is what Cloak is good for - lots of help doing something that's already sneaky.
3. If the Cloak user attacks or does anything else remarkably obvious and unwanted, then the Cloak does not count in any way.