Magento – How to define if url exists at Magento

If I understand your post correctly you are looking to uniquely identify any given requested resource, not just CMS pages, right?
As you've found out, in most cases that works fine, except that for the CMS pages where the page_id param isn't added to the request path info.

The following should suffice in most cases, though you might want to customize it for layered navigation query arguments and such.

public function getRequestIdentifier()
{
    $req = $this->getRequest();
    $params = array('page_id', 'id', 'category', 'p', 'limit', 'order');
    $parts = array(
        $req->getModuleName(), $req->getControllerName(), $req->getActionName()
    );
    foreach ($params as $p) {
        $value = $req->getParam($p, false);
        if (false !== $value) {
            $parts[] = $p;
            $parts[] = $value;
        }
    }
    return implode('/', $parts);
}

In addition to changing the URL which is being rendered, you'll need to rewrite Mage_Catalog_Model_Layer_Filter_Attribute::apply():

/**
 * Apply attribute option filter to product collection
 *
 * @param   Zend_Controller_Request_Abstract $request
 * @param   Varien_Object $filterBlock
 * @return  Mage_Catalog_Model_Layer_Filter_Attribute
 */
public function apply(Zend_Controller_Request_Abstract $request, $filterBlock)
{
    $filter = $request->getParam($this->_requestVar);
    if (is_array($filter)) {
        return $this;
    }
    $text = $this->_getOptionText($filter);
    if ($filter && strlen($text)) {
        $this->_getResource()->applyFilterToCollection($this, $filter);
        $this->getLayer()->getState()->addFilter($this->_createItem($text, $filter));
        $this->_items = array();
    }
    return $this;
}

$this->_getResource()->applyFilterToCollection($this, $filter) is where the ID is passed in. Your change needs to map the option text to the ID prior to this. Something along the lines of the following will get you going:

public function apply(Zend_Controller_Request_Abstract $request, $filterBlock)
{
    $filter = $request->getParam($this->_requestVar);
    if (is_array($filter)) {
        return $this;
    }
    $filter = $this->getAttributeModel()->getSource()->getOptionId($filter);
    $text = $this->_getOptionText($filter);
    if ($filter && strlen($text)) {
        $this->_getResource()->applyFilterToCollection($this, $filter);
        $this->getLayer()->getState()->addFilter($this->_createItem($text, $filter));
        $this->_items = array();
    }
    return $this;
}

I'm a bit concerned as to what effect this will have on caching, etc. but at least this will head you down the road to a solution.