And maybe the bigger problem here is that I wasn't able to stop the auto-landing command. With previous firmware versions, you could cancel it with upward stick movement.
In summary, your incident seems to be the result of the combination of two separate issues:
1. The Mavic going into forced landing from a very high altitude. This is caused by incorrect VPS data telling the Mavic that it's close to the ground when it's not. There's a long thread about it on the DJI forums, but the issue was supposed to be fixed in .0400 of the firmware. The fact that you've had this problem with the newest firmware seems to indicate the fix was not good enough. I would highly recommend you go over to that thread and let the moderators know that you've just experienced this same issue. (Reply to one of DJI-Ken's posts in the thread to make sure he gets alerted straight away). Here's the thread: Mavic Pro will NOT descend unless VPS turned off. Also, if you haven't already done so, do start a claim with DJI support so that they can analyze the incident.
Somewhere in that thread I suggested switching off Landing Protection if your Mavic has this VPS issue, to prevent it entering a forced auto landing.
2. Being unable to abort the forced auto landing. As others have said, this is a new (and apparently intentional) "feature" in the .0500 firmware. I made a thread about it not too long ago. I would also suggest you comment on that thread to let DJI know that this "feature" was partly to blame for your incident, and that they should really reverse the change and give us back a way to cancel a forced landing. Here's that thread: Tip: How to Cancel Auto Landing in v01.03.0500 (DJI, please Fix)