Hmm. So I delayed the meeting for a while by locking the mayor in his room. The mayor, who is in the military, eventually got called to training. I let him out, thinking that his training would prevent the meeting. It didn't. Fortunately, this time I was able to deny it without the game crashing. I kept a copy of the original save file I loaded today, so I'll be able to report this bug.
This immediately caused everyone who was part of the heist crew to leave the fort as they were hanging around in the tavern, singing songs and dancing. Shortly after the first one got off-map, he was caught sneaking around the entrance, just like last time. I decided not to attack him, but the soldiers at the gate did so anyway. Of course, he got torn to shreds.
Then, all of the heist crew members who were still on their way towards the map edge became hostile and started attacking dwarves. This seems to imply that the skirmish that erupted earlier was not a loyalty bug. The reason I thought so is because a long, long time ago, I was messing around with dfhack, and I caused my parent civ to siege my fort. Immediately when those guys entered the map, *everyone* in the fort started attacking each other. This time, I figured that attacking a fellow Dwarf, who presumably happened to be a part of my civ, caused the same thing to happen.
But now I think that the heist "turned loud" when one of them got attacked by the authorities, and all of the other crew members became violent as an intended part of the plan.
I'll see if I can't kill all of them, and if that won't be enough to stop more thieves from coming for it.
The meeting guy also mentioned the name of the person who sent him, so if I fail to protect my artifact, maybe one day I will retire the fort, and steal it for myself in adventure mode. It's a frilly pig tail skirt, so my femboy adventurer will look great.