It's no need to stay at just "thinking" ... you have access to the log & can easily verify all that.
The uncommanded speed increase happens at 19,6sec ... compare the red speed graph with the dashed stick graphs (value 1024=neutral)... & as seen, the coming voltage drop isn't a trigger instead it's a consequence of the rapid speed increase.
View attachment 144720
As the PhantomHelp CSV doesn't include the current ... below is from the AirData CSV instead ... the same here regarding the current, it's a result of the speed increase & isn't triggering the event.
View attachment 144721
That's correct ... the GPS level reaches 4 (of max 5) & a HP is recorded at 17.9sec & the incident starts at 19,6sec. Furthermore a bad GPS reception doesn't cause IMUYaw to be off by approx 90 degrees... at most it will trigger ATTI which will cause horizontal drifts... but without any effects to yaw.
You can't just disregard the very clear facts the log & video together provides ... it's crystal clear that the IMU have wrong information about where the drone points.
Have cut & pasted a bit below to show the difference between what the IMU think's regarding the yaw direction compared to what the video (reality) says about the yaw direction.
The original sat pic was black due to that a mountain peak is shadowing the flight location ... so difficult to judge how the flight path is located in relation to the river.
But could find a better one from Google Earth ... on which I copied in the logs path & the drones yaw direction (green bar). This shows a yaw direction in a south westerly direction.
But looking in the video at the time where the incident starts ... it shows a real yaw direction to south east.
View attachment 144722
So it's no question mark regarding what caused this ... yaw errors on 90 degrees will cause flyaways as soon as the IMU tries to hold position.
The only unknown in this stage is the cause of the yaw error ... power on in a magnetically disturbed location or a IMU failure? And as the DAT log isn't available we can't say more than this.