Re: Extra Time and Constant/Instant Powers
If they only have to do gestures, incantations, concentration, etc. to start a power, you use the instant value. For extra time, this would eb the reduced "only to activate" value.
If they have to maintain those (keep chanting, concentrating, gesturing, ....) the whole tmie the power is active, it's worth more (typically 2x the normal value), except for extra time. The default mode for extra time is that every use of the power takes the extra time.
In your example, every action taken while invisible would take extra time, unless it were purchased as only to activate. (normally maintaining a power is an 'action that takes no time', the limitation changes it to whatever level the Lim is bought at).
In short, the less valuable limitations only affect the initial activation, while the higher value ones affect every use of the power.