Usually the brightness keys should control the backlight and not interfere, but it sounds like they are doing something else.

You can try right-clicking on the desktop and choose "Intel Graphics Settings". Look in Power and see if any of the settings there make a difference. (Personally I always turn off dpst because I don't like how it looks).

There is also a slightly newer driver for the Intel 620, not sure if that would help or not:
https://downloadcenter.intel.com/product/96551/Intel-HD-Graphics-620

A work around I've found is another "last chance" to update to Windows 10.

Windows 10 - Assistive Tech Upgrade

This is a lot like using a sledgehammer to hang a picture but its all I could find. Thanks anyway herf! Keep up the good work.

DisplayLink, it's not for everyone...