Add an option to fork a reusable block to a non-reusable inline-block (and optionally delete the reusable block IF it's not used elsewhere

Huge +1, but I think that adding a delete option may slow things down a bit.

Very rough attempt at something like this! Probably riddled with gotchas and short-sightedness, but I wanted to continue to conversation!

<?php

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Url;

/**
 * Implements hook_form_alter().
 */
function lb_block_fork_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id === 'layout_builder_add_block' || $form_id === 'layout_builder_update_block') {
    /** @var \Drupal\layout_builder\Form\ConfigureBlockFormBase $configure_form */
    $configure_form = $form_state->getFormObject();
    $component = $configure_form->getCurrentComponent();

    $config = $component->get('configuration');
    if (isset($config['id']) && str_starts_with($config['id'], 'block_content:')) {
      $form['#prefix'] = '<div id="fork-block">';
      $form['#suffix'] = '</div>';
      $form['settings']['fork'] = [
        '#type' => 'container',
      ];

      $form['settings']['fork']['notice'] = [
        '#type' => 'html_tag',
        '#tag' => 'strong',
        '#value' => new TranslatableMarkup('This is a reusable block, create inline copy?'),
      ];

      $form['settings']['fork']['action'] = [
        '#type' => 'button',
        '#value' => new TranslatableMarkup('Copy'),
        '#ajax' => [
          'callback' => '_lb_block_fork_callback',
          'wrapper' => 'fork-block',
        ],
      ];
    }
  }
}

function _lb_block_fork_callback(array &$form, FormStateInterface $form_state) {

  /** @var \Drupal\layout_builder\Form\ConfigureBlockFormBase $configure_form */
  $configure_form = $form_state->getFormObject();
  $section_storage = $configure_form->getSectionStorage();
  $section = $configure_form->getCurrentSection();
  $component = $configure_form->getCurrentComponent();
  $config = $component->get('configuration');

  $parts = explode(':', $config['id']);

  /** @var \Drupal\Core\Entity\EntityRepositoryInterface $entity_repository */
  $entity_repository = \Drupal::service('entity.repository');

  /** @var \Drupal\block_content\BlockContentInterface $block */
  $block = $entity_repository->loadEntityByUuid('block_content', $parts[1]);
  if ($block) {

    $clone = $block->createDuplicate();
    $clone->setNonReusable();

    $component->setConfiguration([
      'id' => "inline_block:{$block->bundle()}",
      'label' => $form_state->getValue(['settings', 'label']),
      'label_display' => $form_state->getValue(['settings', 'label_display']),
      'provider' => 'layout_builder',
      'view_mode' => 'full',
      'context_mapping' => [],
      'block_revision_id' => $block->getRevisionId(),
      'block_serialized' => NULL,
    ]);

    /** @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $tempstore */
    $tempstore = \Drupal::service('layout_builder.tempstore_repository');
    $tempstore->set($section_storage);

    $form_state->setRedirectUrl(Url::fromRoute('layout_builder.update_block', [
      'section_storage_type' => $section_storage->getStorageType(),
      'section_storage' => $section_storage->getStorageId(),
      'delta' => array_search($section, $section_storage->getSections()),
      'region' => $component->getRegion(),
      'uuid' => $component->getUuid(),
    ]));
  }

  return $form;
}