Did ever a member of an Illustrious (*Erlaucht*) mediatized comital house marry into a reigning royal family?

Hardly into a reigning princely or ducal family, I think.

Some more examples of such marriages from various other families after 1806:

Her Illustrius Highness Countess Maria Anna von und zu Trauttmansdorff-Weinsberg and Prince Ludwig von Sachsen-Coburg und Gotha

His Illustrius Highness Count Franz Xaver von Stolberg-Wernigerode and Princess Barbara of Two-Sicilies

His Illustrius Highness Count Eberhard von Stolberg-Wernigerode and Princess Marie Reuss zu Köstritz

His Illustrius Highness Count Alfred von Stolberg-Stolberg and Princess Auguste von Waldeck und Pyrmont

His Illustrius Highness Count Bernhard von Stolberg-Stolberg and Archduchess Hedwig of Austria

His Illustrius Highness Count Georg von Waldburg zu Zeil und Hohenems and Archduchess Elisabeth of Austria -Tuscany

His Illustrius Highness Count Georg von Waldburg zu Zeil und Hohenems and Archduchess Gertrude of Austria -Tuscany

His Illustrius Highness Count Eberhard von Waldburg zu Zeil und Wurzach and Princess Maximiliane von Hohenzollern-Hechingen

His Illustrius Highness Count Friedrich Magnus zu Solms-Wildenfels and Princess Antoinette von Schwarzburg-Rudolstadt

Her Illustrius Highness Countess Monika von Solma-Laubach and Prince Ernst August von Hannover

His Illustrius Highness Count Hans von Solms-Baruth and Princess Karoline Mathilde von Schleswig-Holstein-Sonderburg-Glücksburg

His Illustrius Highness Count Hans Veit zu Törring-Jettenbach and Duchess Sophie in Bayern

His Illustrius Highness Count Friedrich von Ysenburg und Büdingen in Meerholz and Princess Marie von Reuss-ältere linie

Her Illustrus Highness Countess Gisela von Ysenburg und Büdingen in Meerholz and Prince Friedrich Wilhelm zur Lippe-Biesterfeld

Her Illustrius Highness Countess Auguste Ysenburg und Büdingen in Philippseich and Prince Heinrich zu Waldeck und Pyrmont

Her Illustrius Highness Countess Pauline zu Ortenburg and Prince Christian zur Lippe

Her Illustrius Highness Countess Ilka zu Ortenburg and Duke Johann von Oldenburg

His Illustrius Highness Count Friedrich zu Castell-Rüdenhausen and Princess Karoline Mathilde von Sachsen-Coburg und Gotha

His Illustrius Highness Count Wolfgang zu Castell-Castell and Princess Sibylle von Reuss

His Illustrius Highness Count Luitpold zu Castell-Castell and Princess Feodora of Denmark

His Illustrius Highness Count Ferdinand von und zu Trauttmansdorff-Weinsberg and Princess Maria Franziska von und zu Liechtenstein

His Illustrius Highness Count Viktor von und zu Trauttmansdorff-Weinsberg and Princess Edina von und zu Liechtenstein

and so on...