I wanted to give an update so this thread can be closed ...
I did have a very long - many days - conversation with Blackview after-sales support team. It was a very positive interaction with a negative outcome.
[plus] the team accepted the BV5000 got an issue, the seller did not reply and accepted I could send it back for warranty repair
[minus] sending the BV5000 from Europe to China and back costs as much as 75% of the price of the phone in AliExpress. Adding custom charges in the way back would make it just not practical to do that.
So the phone is broken and cannot be fixed .... I'm on my own.
What I now did was to first perform a full eMMC memory clear from the Factory Test menu and performed all systems factory tests individually ( to find that menu, start from power-off and push the [power + volumen down] buttons for 3 min) . The tests showed the hardware issues on my phone are around the G-Sensor, the M-Sensor and the "idle-current" detection.
Well, now I rooted the phone, and I hope with a bit of good programming luck I will manage to circumvent those hardware issues.
Thank you to Z. from the After-Sales team for been patient and helpful.
For now this thread is closed.