Get a manual list of unique codes which can be used only once

Is it possible to have a unique Access Code field to allow for viewing/completion of form, but the code would have to pulled from a list of unique codes only one time for each user?

Option 2 - If users can request an access code and the ability to change the code easily for each request?

You can try using the Spreadsheet to form widget, using this widget, it can auto-fill fields with information coming from a list of your uploaded spreadsheet to your form. You can check this guide for more information: https://www.jotform.com/help/442-How-to-Use-the-Spreadsheet-to-Form-Widget

Next, then assign one field to be handling your unique question setting such as your access code field. This will prevent access codes to be used twice in your form. You can check out this guide to know more: https://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question

On your option 2, I'm not sure I'm fully understanding the process. But if you are looking for a way to run a 2-way authentication method, I'm not sure it is possible with JotForm. Basically JotForm is designed to accept and collect data submissions made by users only and not to use it for authentication process. But if I am misunderstanding your concern, please let us know.

Thanks.