Seat Alhambra engine trouble

Sounds familiar. I had a similar problem with my Alhambra. Check engine light, rough idling. Did you check the air filter? Sometimes a clogged filter can mess with the sensor readings.

The air filter appears to be in reasonable condition. Could the mass air flow sensor be defective even if the air filter is not completely clogged?

Yes, the sensor itself can definitely fail. A faulty sensor will give incorrect readings to the engine control unit, even with a clean air filter. Given all your symptoms – engine stalling, check engine light, smoke, and performance decline – a defective mass air flow sensor is the most likely culprit. I'd suggest getting it checked out at a workshop.

Thank you for the insights. I took it to a workshop, and they confirmed it was the mass air flow sensor. The sensor failure was indeed the cause of the performance decline and engine stalling. Repair cost came to 365€. I will also replace the air filter. Again, thank you!