I know, it isn't impossible that there are two different Queen Ethenielles quite close together.

However, I think it makes more sense that it was rather RJ's intention to mention a familiar character in the introduction of SaSG to make it possible for the readers to approximately place the writing of SaSG at the time of the beginning of the series or a few years before. Plus, also in the WoT roleplaying game, which IIRC was supervised by RJ, "Ethenielle Kirukon Materasu" is mentioned as the name for the current Queen of Kandor.

If there is a mistake, it's rather in the Guide, because it came out earlier. Maybe RJ even wanted to change her name to link it better to the beautiful soldier queen of the Trolloc Wars Kirukan, whose ancient Sword of Kirukan is one of Ethenielle's regalia.