I did a clean install.
Unzipped the latest build, renamed the .rockbox folder on the DX90 to XXX, and copied .rockbox from the latest build onto DX90.
Rebooted and ... still have the problems (the battery icon shows as plugged in, whether it is or not. - I have to change defaults to get to the other problems, but they still exist when I do.) As far as the backlight exemptions - the volume, seek/skip keys do work as they should, it is just the play/pause key that always lights up the backlight. It does this even in the August 2020 build.
The rest of the issues go away when I install the August 2020 build, or a June 9 2015 build I originally was using.
I did notice that in the 2020 build, when plugged into USB power and the USB option set to charge only, the battery icon shows as charging (with a sawtooth line in the icon) and this does not happen in the latest builds.