I think I understand what you’re trying to accomplish, but correct me if this is off base.
You have an array that being specified in another file. You want users to be able to append values to that default array by using an options form.
Here’s what I would do:
When you’re preparing this array in your other file, merge your default array with whatever the user sets, if anything. That way the user will never be able to modify your initial array, but can still contribute to it.
Next, you’ll use the options form to store the user array to the database. The final product will look like this:
<?php
// Your preset array. These are your default values.
$whitelist = array( 'your items here' );
// Get the user array from your options.
// This is where the options form stores these values.
$mp_settings = get_option( 'mp_settings', false );
// Check to see if you have any values, if not set an empty array.
$user_whitelist = isset( $mp_settings['mp_textarea_field_0'] ) ? $mp_settings['mp_textarea_field_0'] : array();
// Merging these array together gives you a unified whitelist.
$whitelist = array_merge( $whitelist, $user_whitelist );
Note: this what you’ll add to your other file, the one you didn’t share in this question.