A quick point on the first post, it's not unusual to borrow a part from another airline. The airline's don't ALL the parts for the entire aircraft at each city.
As for the second post, this can happen. The airline has to look at both flights as a whole picture. They may only have one flight per day to Zurich, and it probably was full, vs your flight to Buenos Aires, which they may more than one, and not all full. It comes down to which way is going to have the least impact on everyone.
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