> the holes in the swiss cheese lined up.

I totally agree with you on that.

> The emergency aircraft couldn't find a free gate, creating a massive distraction for ATC, airport, et al.

Yes. And I want to add one more thing to this: the airplane with the "odour" issue was kinda ambivalent about the danger. They deemed it dangerous enough to declare an emergency, and request a gate then later ask for airstairs but not dangerous enough to pop the slides and just evacuate right there and then. I'm not saying this is wrong. Obviously they were evaluating the situation as new information was coming in. But it increased the workload of the ATC. They were trying to find a gate, and etc. If it was a clearer "mayday mayday mayday, aft cabin fire, we are evacuating" that might have been paradoxically less "work" for the ATC. Or at least more of a "practiced" scenario.

> Perhaps it was some other misunderstanding of how that system works.

Yeah. That's a big one. Total speculation but maybe they thought the airplane with the "odour" issue was keeping it red?