Is there a way that I can directly view/download the files for installed systems – for example, if I wanted to compare the system .mjs file currently installed on the Forge to a local version?
Once you’ve uploaded a custom system you won’t be able to download it again, it stays the way you uploaded it.
You can upload a new version of the system again to update it / replace it.
If you haven’t edited the system at all, you should be able to go to the github of the package and have a look at the release. That would be the way to download it too.
Note that you can also access any files directly via its url, like https://mygame.forge-vtt.com/systems/dnd5e/path/to/file.mjs for example.