Are you certain? I believe he is always sitting on a table in the large room behind the throne.
Edit: Just went in-game and confirmed. Helseth is sitting on the table in the large room behind the throne. You have to go through two sets of large wooden double doors. I can't think of any instance where he disappears throughout the game..
Okay,thanks for the help,didn't want to go through any of the doors in case those guards came after me. yes, he was sitting on the table, but I got nothing when I clicked on him in dialogue mode, so the letter didn't come from him.
UESP says this about the marriage card:
"At some point in the game after you complete this quest and if your reputation with Morgiah is greater than 10, you will receive an engraved card from someone dressed in the livery of Wayrest. When you go into your inventory, you may not be able to find it. This is actually an invitation to Morgiah's Wedding ceremony. Don't worry though, you can't actually 'attend' the ceremony so missing it means nothing to the Main Quest."
Now I know I get +10 for delivering the letter to the King of Worms, and I have not done anything else for her, so my reputation with her should be 10,but not 'more than ten'.
My message definitely was the standard 'boy gives you a letter and runs away' type pop-up. Was not described as the above UESP quote at all.