Hi,
I would like to confirm your issue for a better understanding.
"4.0 would roll back after trying to install"
You have removed all Net.frame work version from 4.0 to 4.8, but failed to reinstall them, right?
Did you checked your system meet prerequisites for installation 4.0?
Such as: Windows Installer 3.1 or later and Internet Explorer 5.01 or later.
"For whatever reason, the .NET installer seems to also access E:\, a completely different partition that's on my 2nd hard disk, reserved for Windows 10."
How did you find it?
As Windows 7 is out of support since Jan. 14 2020, did you activated system with ESU(Extended Security Updates) product key?
Any related error message recorded in Event Viewer\Windows logs\Application, system?
Bests,
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