@Tardigrada: Concerning the channel/frequency mode - I encountered the same problem after setting in Parameter general settings the channel mode (displaying channel name in radio). After setting the frequency mode, I was able to switch back to VFO again, but the channel names are of course not displayed, just frequencies.
Additional information to everybody: there seems to be a newer version of firmware available at (link removed after discussion with forum admin, please refer later post for a copy on my gdrive).
To switch the device to firmware upgrade mode, I found out by experimenting it must be turned on when pressing # at the same time (the display stays black and programming is possible using CPS tools menu). There seems to be addition of APRS into radio settings, GPS display contains more information (elevation, sats, speed, …), but some Chinese font characters seem to be missing (black squares are drawn instead). Also, the graphical boot logo is just a white screen and elevation of GPS is calculated/displayed wrong in the GPS overview screen.
I was able to extract the factory firmware (1702.02.v014) without bootloader (and flash it back) and I am 95% sure it is correct and OK, so if somebody needs it I can post it here.
As additional information, powering on the radio with bottom side key (down) pressed shows password prompt which when correct password is entered displays Chinese factory calibration menu allowing to change offsets, output power and some other things (as it transmits on non-free frequencies in some submenus, and changing some settings may make the radio unusable, I will not include the password here but for those who need it, it is visible as plain text string in linked firmware, starting with 6666).