I can help but wonder why Archs Plan (relayed by the surviving passenger) to "climb 1000m and fly directly to Salzburg, as there would be no obstacles in that altitude" didn't work out.
Even if the Garmin had issues (passenger claimed to have seen "no signal" on the portable navigation device) the compass outght to show north quite reliably. Acceleration should not affect the compass when flying north or south, right?