I think there could still be potential liability under Article 19 of the Montreal Convention:
Article 19 — Delay
The carrier is liable for damage occasioned by delay in the carriage by air of passengers, baggage or cargo. Nevertheless, the carrier shall not be liable for damage occasioned by delay if it proves that it and its servants and agents took all measures that could reasonably be required to avoid the damage or that it was impossible for it or them to take such measures.
http://www.jus.uio.no/lm/air.carriag...montreal.1999/
https://www.iata.org/policy/Documents/MC99_en.pdf
But then I think EK or ET could introduce arguments for it to require litigating (was it really a delay, or a cancellation of service causing a passenger to be delayed; were all the hotel arrangements etc. consequential losses etc. etc.etc.) and that's after they pass the buck between them!
I do agree that the EC 261/2004 claim would be a stretch.