Should I submit my Subject Test in Literature score?

It really depends on your major at the end of the day. If you want to major in something English/Literature-based, then you should submit it if you have no opportunities to retake it or if you can, definitely try to retake it again to aim for a higher score on your subject test so that it demonstrates that you understand the subject & strengthens your application and will be under favorable consideration assuming that all your GPAs and extracurriculars on your applications are good. If that isn't your intended major, then you don't have to submit unless you really want to because I don't think there's any hurt in that. Best wishes to you!

It depends. What's your future major? If it includes heavy reading or English, then I'd try to retake the test and submit the best score. Northwestern is a more STEM oriented school, however, so I'd encourage that you take a few SAT Subjects in STEM (think 1 math and 1 other STEM test in your major, if applicable). Having high SAT scores shows colleges you are proficient in an area, so if your school doesn't offer a foreign language you want, then taking an SAT Subject test can show that.

Basically, retake the Literature test and get a higher score and take a few more to further strengthen your application (and to make up if that Lit. score isn't raised.)