Unsure what the issue might be but you might get more traction if you moved the question to a new thread as this one mainly focuses on the Unreal Engine Installer.
When you run the DirectX install that was linked in the workaround steps, what does the installer tell you? I’m having a hard time picturing what you mean when you say this:“My computer tells me I have DX11 installed and won’t let me upgrade”.
No matter how many times I run that installer, it always proceeds.
What build of DX11 do you have installed?
Are your graphics card drivers up to date?
A dxdiag may tell us if your hardware meets the minimum requirements.