The reason the lookup prefers FS over DB is that I view FS-based installers mostly as a dev convenience since it's easier to quickly change file-based installers. I agree that for prod deployments you'd mostly change DB-based installers.

What's implemented is pretty much your second list of priorities - _installers_ in the FS take precedence over _installers_ in the DB. When looking up templates they look in their respective places, but both also look in the 'common' directory on disk.