Is it possible to stop a webform form during submission?
First is I'm not sure if it's best practice to do validations in the submit handler because as you've said, you need to validate first. So I can only answer your second question.
To make your validation run after every validation present for that form, you need to add it in the end of the #validate attribute. Example is like:
$form['submit']['#validate'][] = 'your_last_needed_validation_func';
To check if there are validation errors before your validation function, you can use the form_get_errors function.