when I'm in the UI, landed cost per line is greyed out, like I don't have permissions to uncheck it (even though I'm in admin). I expect there's something about that field that natively you're not allowed to uncheck for whatever reason?

hmmm how come that checkbox is editable in my end

The Landed Cost Per Line fiel will only be editable on create. Once you save the IR, it will be disabled on edit mode.