Thank you flyerCO for the reassurance!

I just wanted to confirm that the erronous segment is now no longer showing (it would've been flown last Saturday), and the rest of my actual reservation is still safe! Hope this helps anyone who ends up in the same situation

As an aside, while this erronous segment was still visible (i.e. after doing the first change, and before "missing" the erronous segment), I wasn't able to view any sensible flight changes (i.e. I'd be shown 3000USD+ flight change options if I tried to change either the erronous segment, or the correct return journey) - now I am able to select just the return journey, and view sensible alternative flights from ZRH-SEA (albeit with a 300 USD change fee ; ) ).