As I understand it, airbag related items are only a failure if the car was registered after they became compulsory, ie if fitted must work/removed is fine for earlier cars and if obligatory must be present and fault free for later ones... Same as rear seatbelts and pre '87? cars

Any Omega seat with side airbags can be fitted to any other Omega but the belt pretensioner must be fitted from the vehicle that the seat is going into. Non side airbag seats can only be fitted to cars without side airbags from new.
Eg:
Pre facelift... Mechanical pre tensioners/no side airbag only suitable for same as a straight swap.
Mfl with mechanical pretensioner /side airbag can be fitted to any Omega but must have electrical pretensioner swapped from original seat if fitted to a facelift.
FL side airbags/electrical pretensioner can be fitted to any Omega but if fitted to pre or mid FL will need electrical pretensioner swapped with original mechanical pretensioner.
Later facelift seats also have active headrests which are sprung loaded to minimise whiplash injury. The trigger is mechanical and load dependant and is in no way connected to the airbag system.
