I booted into my 7 partition the other day because for whatever reason my frayed 10 partition wouldn't let me run an offline defender scan.

My suggestion is, install Microsoft Security Essentials (you can get a copy of the installer out of archive.org) and definition files are still published for it and are up-to-date. MSE is what became Defender in 10. The Defender in 7 is not the same and doesn't scan for the same things. MSE is needed to replace it.