It is possible to do a compass swing in the air, involving many headings to various landmarks across a pinpoint. The Beaufighter procedure mentioned wasn't a compass swing, just an instruction in Pilot's Notes for the type:
Firing the 20mm guns causes disturbance of the P4 compass. It may be restored … by firing a one-second burst (about 10 rounds per gun) while flying level on compass North.
If this was not practical, perhaps because half the Luftwaffe was after you, the notes suggest the pilot uses headings passed from the observer's compass which was not affected.
Further to the sad story of the Australian Proctors, we had to scrap one of these shortly before we chopped the Messenger. I have since recalled conversations with our ARB Surveyor Mr. C. H. Taylor, whose main job in the war years had been as a de Havilland engineer responsible for keeping scores of Tiger Moths on the line each morning.
The problems with casein glue were well known and it was no surprise that the Proctor structures were delaminating as it was kept outside for some time in its history. I remarked that even modern glues could not be trusted as I had read of Mosquitos losing wings in the tropics but Mr. Taylor said there was no problem with the Mosquito adhesives. In fact de Havilland soon found the failures were due to manufacturing defects, but the 'tropics failure' story was allowed to stand in order to preserve morale.