How to save validated third party API data if form validation fails
It sounds like before your mymodule_form_validation($form, &amp;$form_state) function returns you want to store the Stripe information in the users $_SESSION array as a valid attempt/data.

You write to user session using the global session value in PHP and Drupal adds it to the Users' DB/Mongo user session info at the end of page execution. For example:

// in your form validation code ...
$_SESSION['stripe_username'] = 'john_doe';


On form load simply prepopulate the form data from $_SESSION. Eg, you're stuffing the content into session because it will be wiped on form reload -- then you're replacing it so a 2nd account isnt created.