I replied to dosdude1 on the other "unsupported" thread - I´m curious why the SSE4.2 contents in the AMD driver would not crash the boot process as it seems to be the case on certain MacBook Pro models with AMD or even Mac Pro 3,1 with those cards. If you boot into the Mojave installer, can you move the mouse pointer to the region of the second monitor? My suggestion of trying HS instead of Mojave was only to find out what´s going on, not meant as a permanent solution. Just to reduce some Mojave related problems on top of the driver/kext issues. Another thought: Have you tried all possible connector outputs? HDMI, DVI, VGA, DP whatever your RX560 features? Even in combination of adapters (DVI->VGA is sometimes a strange remedy to some boot-related issues). Monoton has nothing more to add, I would ask?!