Reduce the resolution of your screen through settings to half, you certainly don't need that much.
You will also disburden the device this way, it will be more pleasant for you to watch and much more readable.

You can't rid of the W/H button on the screen, it's a matter of the UI and Samsung doesn't allow it.
You can try some third-party software, but it usually doesn't work in cases like this. Maybe with some custom launcher.

Everything you asked has nothing to do with navigation
Try to google a bit in cases like this, the problem is the SYSTEM.