Sorry that I'm not able to move this thread ...maybe someone will help you out soon with that.
In the meantime:
For the Fn keys: you should probably be OK by going to your manufacturer's support website, and seeing if they have a suitable Win10 utility for the Fn keys (I'm assuming you have a laptop). If they don't have a Win10 version, it probably won't kill anything to reinstall an 8.1 version ... they are just fairly simple utilities.
For the screen brightness/power saving: I prefer to use the native Windows 10 power options rather than manufacturer-supplied power utilities... My main reason for this is that the manufacturer's utilities often end up proving redundant - and not in a helpful way ("too many cooks spoil the broth"). Windows has a rather large assortment of options for power savings - covering most any aspect you can think of. You can set the percentage of screen brightness levels for various situations, manage the sleep/hibernation/fast startup/full shutdown/network card power usage/hard drive power usage/ .... & the list goes on...
If you get your Fn keys working again, you'll likely regain the ability to raise or lower brightness using the appropriate Fn keys. To get the brightness that you want every time you start your computer, see if trying the brightness setting in Windows 10 Power Options gets your laptop off to the right start. [I generally uninstall the manufacturer's power utilities.... but you can leave them installed & simply not use them - while you see what works best).
Let us know what works, or if we need to try something else.
P.S. .... late edit: your manufacturer's Fn keys utility might be called a "hot keys" utility.