Correct, PN, the FG1 had no inertial nav, the FGR had an INAS (inertial nav attack system). There were numerous types of align available, two we used (Normal and Quick Action), the third was the one I think you mentioned, PN, which was the 5 minute align giving degraded accuracy due to a drift rate of 5NM/hour.
A normal align took about 12 minutes and was the most accurate at better than 3NM/hour. Clearly that was much too long for our scramble times. So for QRA we would use a quick action align which, basically went like this. When we put the jet on QRA we'd do a normal align and then store the heading, present position, etc, locking the platform (using the laps test button, strangely) so that it was effectively aligned.
External power kept the platform heaters on and maintained it at or above 35 degrees, with the INAS in standby. The aircraft could not be moved now. As the pilot started the first engine on external power the nav could start the QA align. Once the heading and present position were checked, navigator selected NAV and again used lamps test to freeze the platform. From there it took 85 seconds to be ready to go. Accuracy should have been the same as for a normal align.