Dont have much experience on alfas, but my previous car a renault megane, had the airbag light on, and went to a dealer. They said it was the ecu also, and replaced it 2 times!
Light was still on... Well I got ****ed cos it was just expense after expense, so I decided to just leave it on. One day while adjusting seats it went off, and stayed off for a long time.
It was the damn connector underneath the seat, the diagnostic doesnt know the fault so it blames it on the ECU or other bull****. Dont get caught in that vicious cycle.
Inspecting my "new" alfa, I found that the connectors underneath the seat are of horrible quality. And true , while fumbling with them a bit, the airbag light came on. Then I replaced the connectors with some proper ones (Its the yellow one), and it stays off now.
For a cheap solution, you can always bind them together with some cable tie , to fix it in a rigid position.