Is it not possible to install unsigned drivers on Windows 7?
You won't get support from Microsoft in doing so, but you'll be able to continue using Windows 7 and will have to find an alternative way of validating the drivers.
I don't know about windows 7, but on win10, one has to 1. generate one's own key 2. sign the driver 3. turn on test signing mode, which puts a watermark in the bottom-right of the desktop background, somewhat like "activate windows" watermark but only on background. Oh, and 1. and 2. are tedious, poorly-documented processes for which microsoft provides almost no support.
There are/will be if there already aren't patches which will disable the checks completely. With win10 it's a moving target, but 7 is not anymore so you don't have to worry about MS reverting your changes in an update.
You won't get support from Microsoft in doing so, but you'll be able to continue using Windows 7 and will have to find an alternative way of validating the drivers.