Hello Concombre,
As far as I'm concerned, the 3-minute cooling down period is to allow an smoother thermal transition between flight to engines off regimes. Certainly the use of full reverse is a demanding task for an engine and should be taken into account, but I'd time 3 minutes either using reverser or not.
Check FCOM3 3.03.25 p1 for more details.
There are AC outputs available in the cockpit. Most aircraft have one AC output near the 121VU CB panel (the bigger one over the jump seat). Depending on the aircraft configuration you might have other(s). Recently saw a aircraft that had one behind the place you stow the briefcase (F/O side).
Beware though - check AC output voltage and frequency before using it!