Sounds like your ECU is fine, they just mean your ECU has logged a fault code, that fault code tells them the bank 2 lambda sensor isn't functioning. A new sensor isnt that expensive to replace.
Lambda sensor will be throwing the emissions off scale so fix that. You could do this yourself if you think your ok to have a go.
The passenger seat airbag can be fixed too, my guess is its the pad in the seat which tells the car someone is sat in the chair, and so turns on the passenger airbag... but a garage may just turn off the airbag, therefore no warning light etc. Cheapest solution... but do you want to be driving round with no airbag. I've had mine turned off in the past. As replacing the seat switch pad can be steep in price.
Cv boot, as you say is easy.