All your questions/assumptions are answered in the linked paper supplementary material: 50hz, 3 axis, only when charging. Accompanied with actual sample plots for various distances from epicenter, showing p waves and s waves.
Recently they had a significant country wide false alarm in Israel at 3AM... There was a emergency alert cell broadcast (similar to amber alert), which caused everyone to move their phone at the same time, which was falsely detected as an earthquake, which caused an Android earthquake alert to be sent to all phones in Israel 30 seconds later. I guess they didn't plan for this scenario
Edit: Arstechina article seems to mention this: "only three were false positives. One of those was triggered by a different system sending an alert that vibrated a lot of phones"