I jogged down to Cathalon to do a quick FedEx mission for the merchant's guild and found the guild door locked! The lock is trivial but, with an NPC right on the other side of the door (and unmoving), it is impossible to open the door without getting busted for "stealing." Is there some clever way around this? Or is the game just screwed up?

I got around this by quickly teleporting somewhere else and back again. This seemed to unlock the door. It was early morning game time, just after it became daylight.

"- do the quest involving a spider"
I could not figure out what you are talking about, even using Google.

#2 involving the figurine - merchant guild members may come out to fight the spider, leaving the door unlocked :

http://www.gamebanshee.com/twoworlds/walkthrough/cathalon.php

I can confirm that teleporting out of Cathalon to any location and back will indeed unlock the Merchants door in Cathalon.

It still works in 2019 - thanks!

Which of the three suggestions in the "solved" message did you use? Teleporting didn't work for me, I tried all times of the day. I did the spider quest. I haven't tried getting caught but ---- I don't believe one has to pay the fine. The "I don't want any trouble" line seems to let you off the hook. Probably a bug. I delivered the first one without letting the Giriza see it but left them read the second. I'm thinking that may be the cause of the problem - did you perchance do this too?

No, I hadn't let the Giriza see it. I wandered around, waited outside the door for several 'game hours' and then came online.

The teleport did work for me.

Oh well, can't win them all! :)