inline_entity_form_form_alter() incorrectly combines form level and button level submit handlers

Forgive me if this has already been addressed, but in the site I was developing using IEF, I ended up with submit handler on the order edit form firing out of order because (for some unknown reason) the Order UI's button level submit handler was being put into the order form's form level handler array and then IEF was combining these all together so that the Order UI handler ended up getting invoked first (which sets the redirect, basically) before the order was actually saved.

In other words, my submit handler array ended up looking like this:

0 => 'inline_entity_form_trigger_submit'
1 => 'commerce_order_ui_order_form_submit'
2 => 'commerce_order_order_form_submit'

Why that Order UI button handler is being put into the form level array is beyond me, but the behavior of this module is incorrect regardless. When a Drupal form is submitted, the form level submit handlers are only invoked if the triggering element doesn't have any button level submit handlers. Now, however, any form including an IEF will process with both its form level and button level submit handlers, even if it results in contradictory or out of order behavior (as in my case).

I think I understand why you've coded this the way you have - it appears you are simply trying to prevent the processing of entity forms via form level handlers and just always use button level handlers, moving the form level handlers to the button level handlers to achieve that behavior. You can probably get away with just checking first to ensure the $submit_element doesn't have any button level handlers before merging in the form level handlers, though I'd ultimately recommend only putting the IEF submit handler in one place or the other.