So, here my latest "latest" experience on how to get the phone to charge somewhat!
From what I have seen, the phone charges with a reduced rate (0.5A) when connected the first time after a power off / reboot. This takes long (in theory 10h from 0 to full ~ 5000 mAh) but is better than not charging at all.
When I disconnect the charger and ... plug it back in, then the phone reverts to the first mentioned 0.07A (trickle) charge which in turn is useless!
I tried flashing back one of the older 5.1 Android ROMs that are available for download on some of the forum post here but I was not successful!
I go to settings | about phone | wireless update | ... at the top | local updates
and from there have selected 2 different 5.1 Android ROMs stored on my local SD Card.
The phone then reboots, indicates updating for some time (5 min?), then again reboots and starts with Android 6 still in place - NO ERROR messages during the "upgrade" ...
Is this an issue with MY phone or can you in general using this method NOT DOWNGRADE an Android version?
Will I have to ROOT my device and do a manual (flash) ROM installation to go backwards?