Event Observer – Observe Product Quantity Changes Only

Until I find the exact problem here is the solution. Instead catalog_product_save_before use the event catalog_product_prepare_save. The downside of this is that the prepare_save event is dispatched only when saving a product from the admin interface or the API. So if you are saving from a custom code this won't get triggered unless you trigger it manually.

I have a hunch that the problem has something to do with the Mage_Catalog_Model_Product::_beforeSave() method. In there there is some processing of the custom options.
But catalog_product_save_before is dispatched after this processing occurs, so while the custom options are processed in Mage_Catalog_Model_Product::_beforeSave() they are actually empty because the event didn't fire yet so they are not added.
If you move the line parent::_beforeSave(); in the method I mentioned at the top of the method, the options are added (still twice, but they are added). I will post more when/if I find the problem.

[EDIT]
Found it. I was somehow right in the lines above.
Like I said the problem is that catalog_product_save_before is dispatched after the custom options are processed. but this is why it doesn't work.
The custom options are saved in Mage_Catalog_Model_Product::_afterSave() by this code:

$this->getOptionInstance()->setProduct($this)
            ->saveOptions();

But $this->getOptionInstance() is populated with options in _beforeSave when the options array is empty in your case. Hence...nothing to save.
If you still want to use the catalog_product_save_before here is the code that should work.

//check that we haven't made the option already
$options = $product->getOptions();
if ($options){
    foreach ($options as $option) {
        if ($option['title'] == 'Auto Date & Time' && $option['type'] == 'date_time' && !$option['is_delete']) {
            //we've already added the option
            return;
        }
    }
}
$option = array(
    'title' => 'Auto Date & Time',
    'type' => 'date_time',
    'is_require' => 1,
    'sort_order' => 0,
    'is_delete' => '',
    'previous_type' => '',
    'previous_group' => '',
    'price' => '0.00',
    'price_type' => 'fixed',
    'sku' => ''
);
//don't use it like this because it has no effect
//$product->setProductOptions($options);
$product->setCanSaveCustomOptions(true);
//use it this way. Populate `$product->getOptionInstance()` directly
$product->getOptionInstance()->addOption($option);
//don't forget to state that the product has custom options
$product->setHasOptions(true);

This is not possible - the event will be fired each time for every module including yours. You can only programmatically deactivate the event observer completely, but this is not what you are after.

So the thing you would normally do is to directly check in your observer method if the cookie is set and if it is not set, return instantly:

public function myObserverMethod($observer)
{
    if (<cookie_not_set>) {
        return;
    }
    // do whatever you need to do
}

I understand your point that you do not want to do it like that, because the event is fired so often. The only solution I see here is that you could potentially use a more specific event which is not fired that often like controller_action_predispatch_cms or controller_action_predispatch_cms_index_index. Have a look at the event dispatching code here in order to find a suitable event.