Hi @RyanFino ...
Full disclosure, I used to work for a Technical College in Wisconsin (it's been a few years since I left there) that had (and I believe still has) Banner as their SIS (Student Information System). While I did not work directly with Banner and know the inner workings of it, as a Canvas LMS admin for that school, I did work with one of our IT guys occasionally when I had Banner-related questions and issues. So, I can (hopefully) help to answer some of your questions.
Is there any functional differences between Banner-fed courses (as in, ones imported automatically from a Banner load) and courses manually created from scratch within Canvas?
Generally speaking, no. There shouldn't be any major differences in a course's functionality when comparing if it was created from Banner or if it was created manually by you. If a new course shell was created from Banner, it most likely will have a some sort of data filled in for the "SIS ID" field within the Canvas course "Settings" page. But, if you are creating a course manually, the "SIS ID" field isn't filled in automatically, and you'd have to populate this. Also, there may be other course "Settings" that come over from Banner such as the sub-account it's placed in, the term that it should go to, start/end dates, and format...as some examples.
Functionally, they should operate the same, right? If we had created a custom role named Librarian in Canvas, that role could be assigned to users within a Banner-fed course and a manually created course, correct? The default roles (such as Observer, Designer, etc.) could have their permissions modified the same way even if our Canvas instance was exclusively using Banner-fed courses, right? The speed-grader permission could be added to the Observer role, for example?
With your custom Librarian role, you can assign that role to both Banner-created course shells and also to manually created courses. For example, at the College where I used to work, we had a custom role called "Viewer" which was based on the "Teacher" role type with a bunch of permissions turned off. We manually enrolled people in courses in our "Master Term" ... these courses were also manually created by my team ... not by our Banner SIS. And yes, you can modify those permissions of the default roles if needed as well.
Additionally, could people NOT imported directly from Banner be added to a course later on? Or will the Banner load remove that person (or prevent it altogether) at the next subsequent load? As in, if we had a Librarian role, could we add someone NOT enrolled or teaching the course into that course with the Librarian role?
Yes, this should be possible. But, just so you can verify this on your own, I would suggest that you set up either a sandbox course or do some testing in your school's Canvas "test" environment. What you could do is enroll someone with the "Librarian" role in a course, and then coordinate with your IT person to send a few users from Banner to that course via SIS import. Then, once that is done, check the enrollments of the course to see if the "Librarian" user has been removed or not. This should give you a good idea of how things would typically work moving forward.
Again, this it's been a few years since I've worked with anything related to Banner, and the setup that I was somewhat familiar with may differ than what you have at Seton Hall University. So, if you are not the IT person who handles data from Banner to Canvas, you might want to start having those conversations.
I hope this info will be helpful for you in some way...for what it's worth.
Take care...be well.