I'm looking to make edits to Exchange rates on Invoices via SuiteScript so I don't have to give users access to edit currencies directly. This is due to a different exchange rate than HSBC or Xignite .

You can change the role under which a Suitelet executes, so that sounds like it would work for you. A client script will always execute with the permissions of the current user so you would still need to give edit access to the user. Depending on your UX goals, you could have the client script make an ajax call to a Suitelet with elevated permissions acting as an API endpoint.

I'd create an "exchange override" field then either on save or another button, update the exchange rate via the suitelet