If it was the immobalizer the car would start and run normally (including voltage to the ECU etc) for 3-5 seconds and then stop, with the IMMOB light flashing.
Your problems likely very are elsewhere.
A full code scan with a VAG-specific scanner like VCDS would be a really helpful next step... the car may already know what's wrong.
Does the glowplug light come on briefly when the ignition switch is first turned to "ON" ?