The thing about airbags is that they make cars safer for the occupants (mostly), this is generally a good thing and a net positive. The trouble is that it then allows car manufacturers to build slightly less safe cars that and still pass crash tests, design regulations and other safety regulations. So the reason that some cars don't have an airbag disabling key in the glove box or elsewhere could be because disabling the airbag would put the car in a state where it would not pass safety regulations so the manufacturer (or relevant government body) decided it would never be a valid option. Of course, another reason is to not fit a disabling key is that its cheaper not to.
This is just a general statement about airbags and the way cars are designed, manufactured and tested these days, it's not specific to the Vauxhall (Opel) Zafira MkII.
Assuming you are aware of all the pros and cons of disabling the airbag (I've only broadly outlined a basic safety issue, there is more than that to consider) your options are very limited because unfortunately you've discarded pretty much everything that is likely to be effective. But I do agree with most of your reasons for discarding them.
Your best bet is to find another dealer that will disable it for you.
Or try to find someone who isn't a dealer but has the equipment to do it for you. Aftermarket shops are often familiar with the concept because they replace airbag steeringwheels with 'race' ones.
As William Cline has suggested in a comment, you could try to source an appropriate aftermarket OBD system that will allow you to turn the airbag off yourself electronically.
Disconnecting the airbag is still an option, it should be a relatively simple task especially if you can get hold of a decent mechanics manual for the car. There are at least 2 Cons to doing this though:
- Disconnecting will definitely cause
a 'fault' to be registered somewhere
in the cars electronics and it will
almost certainly cause a warning
light on the dash to be illuminated.
In more modern cars, often once an
Airbag fault is recorded the warning
won't be turned off until a dealer
resets the 'computer' so even if you
plug the airbag back in again, the
warning light will likely remain on.
- You probably won't know 100% for
certain that you have correctly
disconnected the airbag and it's not
something easily tested.