Good Day. A warm welcome to the HP community. I reviewed the case regarding issues with booting up the HP slate 7. I will be delighted to assist you here.
Superb description, brilliant troubleshooting and terrific observations made before posting. Kudos to you for that. 😉
To restore the tablet to working condition when this error screen appears, press and hold the power button for more than eight seconds. After the tablet fully shuts down, wait a few seconds, and then press the power button to turn on the tablet. If the tablet boots normally and the error screen does not reappear, you are done. If the error screen reappears, perform a factory reset on the tablet.
Just wanted to add that if push comes to shove and if one has to do a factory reset, one will lose any unsaved data or after boot added apps. It's always wise to have a system back up either on the cloud or Google backup settings when one creates thier google account.