I want to rule out DisplayFusion version 8.1.2 as being a part of a problem I'm trying to resolve with my Windows 10 PC. Can I just disable DisplayFusion without uninstalling it?

Kind of like bring the PC up in safe mode, but in "No DisplayFusion" mode.
I know I can uninstall, but I don't want to have to re-install it and so on.

Yep, just exit out of display fusion by right clicking the tray icon, and selecting exit. (this will close display fusion)
alternatively (and probably better if you are troubleshooting) would be to uncheck 'start with windows' (very first option in the settings) then restart (this should prevent display fusion from even loading)

Super. Thanks.