Hi,
After a long and arduous journey, I finally got a new glow plug control unit.
VW: 125 Euros.
It appears there's a bottleneck, with the delivery date from the central warehouse in Kassel being two days away, on April 4th. Fortunately, the parts service, located 60km away, found a small VW dealer who had the part in stock.
Plugged in and it's already diesel-ing again *happy*
The new part has hardware version 14 and software version 14.
"Error entries deleted. I'll keep an eye on the issue with the undervoltage."
Fun fact aside: my now ex-VW master actually told me that the control unit was defective because I hadn't maintained my glow plugs

. When I asked how to maintain them, he said, "Just use blue or yellow diesel, then they don't need to glow as much, and the control unit won't break either"

.
He probably wears a tin foil hat in his free time.
Thank you for your help, and good night.
Faxes.