Features - Custom User Fields Not Available after Import
Solution

You have to add the corresponding field instances in feature as well. The field instance name should be in this format:

[entity_type]-[bundle]-[field_name]

In your case, they should be:


user-user-field_first_name
user-user-field_last_name
user-user-field_title
user-user-field_email




Sub Question 1


  Despite this, when going to Configuration > People > Account settings, going to "Add existing field" does not list the newly imported fields to that list. They also are not available as "Existing Fields" when modifying a Content Type.


Answer

You cannot find existing fields because there is no field instance attached on the field base. Drupal won't be able to load those field base without field instance.



Sub Question 2


  On top of that, when attempting to create a field with the same machine name, it states that it is already taken.


Answer

The field base is stored in database and also the table is created. So Drupal will stop you from recreation.