I'm not sure which of your three issues to address
1) not displaying anything
2) shows boot screens and goes black
3) catalyst installed which means you have OS screen but get an error message
so here's some things to try and consider:
You don't need video drivers to get an OS screen. You can remove all driver remnants with DDU in safemode if necessary then install 17.2.1
Stick with one monitor into the GPU or into the motherboard. NEVER BOTH.
Check your BIOS setup to make sure it knows which of those two to use as primary video (onboard or discrete). Set it to the discrete gpu.
Use DB25 connection if DVI isn't working or vice/versa.
If no luck make sure BIOS is set correctly then..
Do the same connection swap using output from Cpu/motherboard gpu.
I can't find specs on your hanns-g monitor so I'm disregarding it.
I've now done the DDU thing and reinstalled the drivers you reccomended and the issue is still there, the BIOS looks in-check too
Which issue? If you used DDU then either you did get an OS screen or you used it off a flash drive. Which?
Safemode for DDU worked OK so your gpu and cable is likely not the problem.
When you put in your windows repair disk how far do you get?
I have the same problem with my R 290X now after updating to 17.3.1. Now I can not install ANY AMD drivers as it results in just a black screen, I have even try it on a completely fresh installation, but still not luck. If you want to read my tale of woe see my discussion below.
About the only thing you both can do is try it in another computer. It still may be your motherboard or the PSU.