I think so many of the worst impacts could have been avoided if EK had pre-emptively cancelled some of their flights based on the forecasts and offered change fee waivers for the day of the rain and the day after.

A lot of other airlines do this. It's unclear why EK does not. As with a lot of things relating to wait times (road traffic, queues, etc) a slight reduction in input can mean a massive improvement in throughput during a capacity shortage. There's entire textbooks written by the queueing theory geeks which explain this in good detail.

EK229 to Seattle has been cancelled for 3 days in a row now.

Most passengers are reasonable, but the communication from the company has been very poor. From what I hear, the customer service people at the airport did not seem to have the authority to do anything despite the extraordinary circumstances.

Hopefully they'll have a solid response plan in place for the inevitable next storm, which could be worse than this one.

Flood water keeps rising long after the rain ends. And desert land is more likely to flood from rain than moist land. This means their response plan has to assume waterlogging both inside and outside the airfield long after the rains have stopped.