Symfony unmapped entity form field not having data

In Symfony 2.6, I am using an entity form type that is unmapped:

$form
->add(
    'myEntity', // Form field name
    'entity',
    [
        'mapped' => false, // Not mapped
        'class' => 'MyVendor\MyBundle\Entity\MyEntity',
        'choices' => $MyEntityCollection, // list of MyEntity
        'property' => 'name',
        'empty_value' => 'Please select MyEntity',
        'empty_data' => null,
        'attr' => [
            'label' => 'My label'
        ]
    ]
);

This allows user to properly select an item of MyEntity or leave it blank. According to that, I am adding a EventSubscriber to modify the preSubmitted data if any value is selected, and leave it as it is if no choice has been made.

Here is the eventSubscriber:

/**
 * {@inheritdoc}
 */
public static function getSubscribedEvents()
{
    return [
        FormEvents::PRE_SUBMIT => 'preSubmitData'
    ];
}

/**
 * @param FormEvent $event
 */
public function preSubmitData(FormEvent $event)
{
    if( null === ($entity = $event->getForm()->get( 'myEntity' )->getData() ) ){
        return;
    }

    // Set value if field has been defined
    $event
        ->getForm()
        ->setData( $entity )
    ;
}

If user selects a choice other than blank, when I debug the preSubmitData function:

• $event->getForm()->get('entity')->getData() gives null
• $event->getData() gives an array having as 'entity' key the selected entity ID (just the scalar value)

My questions are:

• Shouldn't $event->getForm()->get('entity')->getData() have the selected entity?
• Why is $event->getForm()->get('entity')->getData() giving null if $event->getData() has at least the entity ID in it?
• Is there any way to get the entity here (as it happens with the mapped entities) without having to call the entity manager and querying the entity via its ID?

Thanks in advance!

Edit

For the big picture, in my global form (other fields not described here) I have 2 depending fields:

• A select A (not described here) with some options from a tree. This option does exist in the global form entity as a property.
• A second B select named myEntity (described here). It doesn't exist as the global form entity as a property, thus the mapped = false. If any choice is made here, then the first select (A)'s option is overridden by this one. Else the first choice remains as the entity property value.

Hope is clearer now.