Ultimately I am looking to create, manage and maintain a #Data_Dictionary for all of my Marketo Fields and demonstrate where they map into integrated Systems such as SFDC and Cloud Apps
Unfortunately, you're looking at this the wrong way for Marketo.
Marketo String fields are not enumerations (= they do not have a defined set of allowed values).
On Marketo forms, you may (at your option) choose a Select dropdown -- or other non-user-editable widget such as Checkboxes or Radios -- to restrict the values a non-malicious user of that specific form can submit to the field.
But that doesn't mean the field cannot hold other values, whether submitted as hidden fields (which are always free text) or via other methods such as the API, or simply via Change Data Value flow steps.
It also doesn't mean another form can't use another type of widget for the same field.
If what you're looking for is more of a cardinality report (= a report of all the unique values currently stored in a field) that's not directly available in the UI, you'd have to export all your leads who have a value for that field, then dedupe on that field in Excel/Sheet/programming language of your choice.