I have a similar issue. The backlight has failed on my mid-2010 iMac 27”. I now have an ultra HD 35” widescreen monitor connected vie display port and that works fine. The only problem is that sometimes I will open an app but it will appear on the iMac screen instead of the external one. This means I cannot see the app!
What I need is a way to make the external screen the ONLY one and not as part of an extended desktop to the internal iMac screen. I have yet to find a way to do this so if anyone knows a way please let me know?