My end goal is to put the Inline Entity Form inside of a custom widget, and have it submittable without submitting the parent form inside of which it has been placed.

I'm not sure how to do this programmatically.  I looked for an example in the tests/ subdirectory of IEF, and found a form like this:

    $form['inline_entity_form'] = [
      '#type' => 'inline_entity_form',
      '#entity_type' => 'node',
      '#bundle' => 'ief_test_custom',
      '#form_mode' => $form_mode,
    ];
    $form['submit'] = [
      '#type' => 'submit',
      '#value' => t('Save'),
    ];

But if I take this code and put it inside an existing form via hook_form_alter or a custom widget, it will submit the entire form, when in reality I just want it to save the values for the reference.  I also tried to experiment by putting a rendered form into an iframe, but it wouldn't load the form code inside it.

    $form = \Drupal::formBuilder()->getForm(\Drupal\user\Form\UserLoginForm::class);

    $ief = [
      '#type' => 'inline_entity_form',
      '#entity_type' => 'node',
      '#bundle' => 'article',
      '#default_value' => \Drupal\node\Entity\Node::load(1),
      '#form_mode' => 'default',
    ];

    $element['inline_entity_form'] = [
      '#type' => 'inline_template',
      '#template' => '<iframe srcdoc="<div id="iframe-ief">{{ ief|escape }}</div>"></iframe>',
      '#context' => [   // Tried the above with & without |escape
        'ief' => $form, //<--tried both $form and $ief here.
      ],
    ];

Is there any example of doing this that I can look for, or any suggestions or which direction to look?