x86 programs WILL NOT close the app gap on Windows Phone, and even considering it as an option is showing a poor understanding of the UWP platform.

This is because x86 apps will not run on Windows Phone at all. x86 apps are compiled for the x86 architecture - since all current lumia devices are using ARM processors they cannot run on these devices. Even if you recompiled the app to run on ARM devices, there's a crap ton of desktop stuff missing from the WP10 operating system, which means they won't run anyway, even in Continuum.

You've probably misunderstood the purpose of Project Centennial, which converts desktop apps to a UWP packaged application. This isn't a simple tool to automatically convert a desktop app into an app that can be run on any device. It just makes the app package into an appx package, which can then be submitted to the store. It also allows the developer to use UWP APIs within the desktop app. It's up to the developers to slowly convert their app to a UWP application. Only when the developer ports everything to the UWP APIs can the app run on anything other than full desktop Windows.